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Thread: Hydraulics myths

  1. #1
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    Hydraulics myths

    I recently needed to re-subscribe to a hydraulics newsletter, and as a consequence I'm receiving back issues.

    The one below took me a while to understand when I first read it a few years ago, so I thought I'd post here (it's short) for others to contemplate. I doubt the author would complain.

    "Hello John,

    In your last hydraulics bulletin, I exploded the myth about pump suction strainers - they do more harm than good. And installing them to 'protect' the pump is a contradiction because they can actually destroy it.
    Here's another big myth ...
    Myth #2. Drift in a double-acting cylinder is always caused by a leaking piston seal.
    A popular misconception about hydraulic cylinders is that if the piston seal is leaking, the cylinder will automatically drift down.
    The reality is, if the piston seal is completely removed from a double-acting cylinder, the cylinder is completely filled with oil and the ports are plugged, the cylinder will hold its load indefinitely - unless the rod-seal leaks.
    What happens under these conditions - due to the unequal volume either side of the piston, is fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the only way the cylinder can move is if fluid escapes from the cylinder via the rod seal or its ports.
    If you grasp the theory at work here, you'll probably realize there are a couple of exceptions. The first is a double-rod cylinder - where volume is equal on both sides of the piston.
    And the second is when a load is hanging on a double-acting cylinder. In this arrangement, the volume of pressurized fluid on the rod side can be accommodated on the piston side. In this case a vacuum will develop on the piston side and depending on the weight of the load, this may eventually result in equilibrium that arrests further drift.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Author of Insider Secrets to Hydraulics
    Last edited by john maddock; 10-04-21 at 09:43 PM. Reason: Clarity
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    Re: Hydraulics myths

    The bit to consider in an effort to understand the principle are the ideals required, ie the gland seal being 100% leak proof and the ports being fully sealed! Not something that often occurs in some systems under working conditions?

    I've found that one trick to use on hydraulics is to imagine what the oil is being made to do internally in components although some systems can be fiendishly complicated and hard to follow.
    A Pea Viner course I went on in the 80's was made extremely complicated by the instructors employed by the manufacturer, at first we all though it was because these fellows from the Nottingham Mining industry were teaching us all about some new radical techniques and we just couldn't grasp what they were trying to get us to understand, then by the 3rd day the pennies could be heard dropping as one after another, we all began to understand where they were coming from and it all fell into place!!

    Of course, now 35+yrs on the complexities of some hydraulic systems have become mind boggling, same go for todays electrical & electronic systems too.
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    Re: Hydraulics myths

    Quote Originally Posted by Footsfitter View Post
    The bit to consider in an effort to understand the principle are the ideals required, ie the gland seal being 100% leak proof and the ports being fully sealed! Not something that often occurs in some systems under working conditions?

    I've found that one trick to use on hydraulics is to imagine what the oil is being made to do internally in components although some systems can be fiendishly complicated and hard to follow.
    A Pea Viner course I went on in the 80's was made extremely complicated by the instructors employed by the manufacturer, at first we all though it was because these fellows from the Nottingham Mining industry were teaching us all about some new radical techniques and we just couldn't grasp what they were trying to get us to understand, then by the 3rd day the pennies could be heard dropping as one after another, we all began to understand where they were coming from and it all fell into place!!

    Of course, now 35+yrs on the complexities of some hydraulic systems have become mind boggling, same go for todays electrical & electronic systems too.
    FF wrote:

    Of course, now 35+yrs on the complexities of some hydraulic systems have become mind boggling, same go for todays electrical & electronic systems too.[/QUOTE]

    Not wrong, FF, although -ignoring the control valves - at least you can trace the flow of oil through pipes. It's just a tad more difficult to trace the flow of electrons in an integrated circuit.

    Here's the next hydraulics myth from Brendan Casey:

    Myth #3. New hydraulic oil is clean hydraulic oil.
    New hydraulic oil straight from the drum, has a typical cleanliness level of ISO 4406 23/21/18.
    Now that number may not mean a lot to you, but it's four cleanliness code levels below that considered ideal for a high pressure, high performance hydraulic system.
    Looking at it another way, a 25 GPM pump operating continuously in hydraulic oil at 23/21/18 will circulate 3,500 pounds of dirt to the hydraulic system's components each year.
    To add hydraulic oil, and not the dirt, always filter new oil prior to use in a hydraulic system.
    This can be accomplished by pumping the oil into the hydraulic reservoir through the system's return filter. The easiest way to do this is to install a tee in the return line and attach a quick-connector to the branch of this tee.
    Attach the other half of the quick-connector to the discharge hose of a drum pump.
    When hydraulic oil needs to be added to the reservoir, the drum pump is coupled to the return line and the oil is pumped into the reservoir through the return filter.
    As well as filtering the oil, spills are avoided and the ingress of external contamination is prevented.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Author of Insider Secrets to Hydraulics
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    At the viner course we had slides showing under a microscope the differences of "new oil" and oil filtered to a higher standard which was very noticeable. Way before the New Viner course the Pea Group modified the old machines by blanking off the filler, mounting a small mini-digger style engine filter right up on the top of the machine, and then using a "revolver" filtering unit to pump the oil into the tank (used a genny out in the field to power it) The revolver was used out of season to clean and condition the oil because it had two large finer filters than the machines own return filters thus improving the oil out of the barrel and later when in service.

    That modification made a huge impact the first year it was implemented. poor hygiene in difficult field circumstances had been a big issue in breakdowns, it didn't eliminate them but it made a huge difference in one simple mod.
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    Re: Hydraulics myths

    Quote Originally Posted by Footsfitter View Post
    At the viner course we had slides showing under a microscope the differences of "new oil" and oil filtered to a higher standard which was very noticeable. Way before the New Viner course the Pea Group modified the old machines by blanking off the filler, mounting a small mini-digger style engine filter right up on the top of the machine, and then using a "revolver" filtering unit to pump the oil into the tank (used a genny out in the field to power it) The revolver was used out of season to clean and condition the oil because it had two large finer filters than the machines own return filters thus improving the oil out of the barrel and later when in service.

    That modification made a huge impact the first year it was implemented. poor hygiene in difficult field circumstances had been a big issue in breakdowns, it didn't eliminate them but it made a huge difference in one simple mod.
    A very interesting comment, FF. I'm not familiar with a "revolver". What is it & how does it work?

    JV
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    Re: Hydraulics myths

    Revolver may of been a trade name for the ones back then in the 80/90's. This is the sort of thing-

    https://hyquip.co.uk/hydac-filtratio...70-m-z-bm.html


    Basically an electric motor & pump, a filter unit, and suction/delivery hoses. Some like we had used spin-on cartridge filters or like this hydac one may have a pot and element type filter. When the Pea viner groups mechanic was doing their winter maintenance any machine brought into the workshop, or even if they were parked in the machinery shed they could be connected up and the filter unit left on to circulate and clean the oil with finer micron filters, plus there was the benefit of pumping oil through one of the filter machines to add/top-up oil- in the field a 200ltr barrel was mounted on the fuel bowser in a cradle, fitted with a male QRC we would use the generator in the maintenance landrover to power the conditioning pump having just wiped and coupled up the hose connectors so no contamination from dirt,dust, mud etc like before and the new oil was also filtered to a better standard than it left the blenders! Hence why problems and breakdowns improved.
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    Re: Hydraulics myths

    Quote Originally Posted by Footsfitter View Post
    Revolver may of been a trade name for the ones back then in the 80/90's. This is the sort of thing-

    https://hyquip.co.uk/hydac-filtratio...70-m-z-bm.html


    Basically an electric motor & pump, a filter unit, and suction/delivery hoses. Some like we had used spin-on cartridge filters or like this hydac one may have a pot and element type filter. When the Pea viner groups mechanic was doing their winter maintenance any machine brought into the workshop, or even if they were parked in the machinery shed they could be connected up and the filter unit left on to circulate and clean the oil with finer micron filters, plus there was the benefit of pumping oil through one of the filter machines to add/top-up oil- in the field a 200ltr barrel was mounted on the fuel bowser in a cradle, fitted with a male QRC we would use the generator in the maintenance landrover to power the conditioning pump having just wiped and coupled up the hose connectors so no contamination from dirt,dust, mud etc like before and the new oil was also filtered to a better standard than it left the blenders! Hence why problems and breakdowns improved.
    Good info, FF. Thankyou.

    Which filters have the best filtering capacity: engine oil or fuel?

    JV
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    Re: Hydraulics myths

    Quote Originally Posted by john maddock View Post
    Good info, FF. Thankyou.

    Which filters have the best filtering capacity: engine oil or fuel?

    JV
    My answer has to be- a good quality one in either application.

    Some people have terrible hygiene standards when it come to decanting oil into engines- dirty jugs and funnels, dirt and dust in with the oil type of thing

    Likewise there are those who push their luck with fuel storage and again like above in how they get it into the fuel tank!

    I suppose the award for best filtering capacity has to go to the paper element filter on old Massey Fergusons like your old beast?

    On the farm where I grew up, the not so bright with modern machinery of the two brothers who owned the farm was renowned for draining the engine oil and filling with new but the with the filter element, being a spend thrift he used to take the element out and wash it in diesel before putting it back in again time after time!
    Eventually there was a change in brand to a Ford 6600 and then he was forever complaining thereafter about the expensive waste of money on the new tractor because he could no longer wash the filter element out seeing as it was sealed in the metal spin on filter casing!
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    Re: Hydraulics myths

    [QUOTE=Footsfitter;312519]My answer has to be- a good quality one in either application.

    Some people have terrible hygiene standards when it come to decanting oil into engines- dirty jugs and funnels, dirt and dust in with the oil type of thing

    Likewise there are those who push their luck with fuel storage and again like above in how they get it into the fuel tank!

    Ha! Reminds me of my fuel tank story!

    Many moons ago, SWMBO had a Mazda 929 car. On irregular occasions, as she was driving to work, the engine would die slowly. First time it happened, the RACT man found and changed the small fuel filter. Second time, she called me. I followed the RACT man's action and (not having a new filter), shook the petrol out of the old one (noting a lot of rust particles) & re-installed it. The car ran fine for quite a while. After a few more events, it dawned on me that the problem appeared only after the car was refueled from the farm's bulk petrol tank within a day of delivery. The delivery stirred up the rust, and the car's recirculating fuel system (it was a carburetored engine, the model before fuel injection was introduced) took about 10 minutes to filter out the rust particles, blocking the small filter and starving the engine of fuel.

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    Re: Hydraulics myths

    In your last hydraulics bulletin, I explained how to add oil to a hydraulic system - without the dirt.
    Today I want to clear up another popular misconception, about the installation of hydraulic components ...
    Myth #4. Because oil circulates through hydraulic components in operation, no special attention is required during installation beyond bolting the component on and connecting its hoses.
    Nothing could be further from the truth, as this example illustrates: I recently conducted failure analysis on a hydraulic motor that was the subject of a warranty claim. The motor had failed after only 500 hours in service, some 7,000 hours short of its expected service life.
    Inspection revealed that the motor's bearings had failed through inadequate lubrication, as a result of the hydraulic motor being started with insufficient oil in its case (housing).
    After this motor was installed, its case should have been filled with clean hydraulic oil prior to start-up. Starting a piston-type motor or pump without doing so, is similar to starting an internal combustion engine with no oil in the sump - premature failure is pretty much guaranteed.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Author of Insider Secrets to Hydraulics
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    Another mythbuster from Brendan Casey - much more esoteric, this time.

    JV

    Today I want to clarifiy another myth, that causes a lot of hydraulic problems - and even failures ...
    Myth #5. All oil returning to the hydraulic reservoir should be filtered.
    True. With one VERY important exception: The case drains of hydraulic piston pumps and motors. Connecting case drain lines to return filters can cause excessive case pressure, which has a number of damaging effects.
    High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, which eventually results in a leaking shaft seal.
    The effect of high case pressure on in-line piston pumps is the same as excessive vacuum at the pump inlet. Both conditions put the piston ball and slipper-pad socket in tension during intake.
    In severe cases this can result in buckling of the piston retaining plate and/or separation of the bronze slipper from the piston, causing major failure.
    Under certain conditions, high case pressure can cause the pistons of radial piston motors to be lifted off the cam during outlet. When this happens, the pistons are hammered back onto the cam during inlet, destroying the motor.
    For the reasons described above, conventional depth filters are generally NOT recommended on case drain lines.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Author of Insider Secrets to Hydraulics
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    Hello John,
    In your last several hydraulics bulletins I've exploded five popular hydraulics myths. And I invited you to get YOUR copy of Insider Secrets to Hydraulics.
    Today I want to switch gears and talk about a major cause of rod-seal failure in hydraulic cylinders.
    As a product group, hydraulic cylinders are almost as common as pumps and motors combined. They are less complicated than other types of hydraulic components and are therefore relatively easy to repair.
    As a result, many hydraulic equipment owners or their maintenance personnel repair hydraulic cylinders themselves. And this is why I included a whole chapter about carrying out effective repairs on hydraulic cylinders in 'Insider Secrets to Hydraulics'.
    An important step in the repair process that is often overlooked by do-it-yourself repairers, is checking rod straightness.
    Bent rods place load on the rod seal causing distortion, and ultimately premature failure of the seal. So rod straightness should always be checked when hydraulic cylinders are being re-sealed or repaired.
    The procedure for doing this is explained in detail on pages 82 and 83 of Insider Secrets to Hydraulics
    In most cases, bent rods can be straightened in a press. It is sometimes possible to straighten them without damaging the hard-chrome plating, however if the chrome is damaged, the rod must be either re-chromed or replaced.
    BUT a word of CAUTION before I go. Attempting to straighten induction-hardened rods can cause the hardened case to shatter - with the potential for serious personal injury and/or property damage.
    For this reason, before attempting to straighten any cylinder rod - make sure it hasn't been induction-hardened first.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Author of Insider Secrets to Hydraulics
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    More from Brendan Casey:

    Hello John,
    In your last couple of hydraulics bulletins, we talked about how to avoid troubleshooting mistakes by checking and eliminating the easy things first. AND I invited you to download my hydraulic troubleshooting app for your smart phone or tablet.
    Today I want to move onto how to determine the condition of the hardest working component of a hydraulic system - the pump.
    As a pump wears in service, internal leakage increases and therefore the percentage of flow available to do useful work (volumetric efficiency) decreases.
    If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.
    In a condition-based maintenance environment, the decision to change-out the pump is often based on remaining bearing life or deterioration in volumetric efficiency, whichever occurs first.
    Volumetric efficiency is the percentage of theoretical pump flow available to do useful work. It is calculated by dividing the pump's actual output in liters or gallons per minute by its theoretical output, expressed as a percentage. Actual output is determined using a flow-tester to load the pump and measure its flow rate.
    Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be stated when stating volumetric efficiency.
    For example, a hydraulic pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5000 PSI and 120 SUS is said to have a volumetric efficiency of 94% at 5000 PSI and 120 SUS.
    When calculating the volumetric efficiency of a variable displacement pump, internal leakage must be expressed as a constant.
    To understand why this is so, think of the various leakage paths within a hydraulic pump as fixed orifices. The rate of flow through an orifice is dependent on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity.
    This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump's displacement.
    A real world example, which shows how costly it can be if you don't understand this concept, is detailed on pages 29 - 30 of The Hydraulic Troubleshooting Handbook

    Yours for better hydraulics knowledge,
    Brendan Casey



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    Re: Hydraulics myths

    The next installment from Brendan Casey:info too good to not pass it on


    Hello John,
    In your last hydraulics bulletin, I explained how to use volumetric efficiency to correctly determine the condition of a hydraulic pump.
    Today I want to continue on this tack and talk about testing hydraulic cylinders.
    The conventional way of testing the integrity of the piston seal in a double-acting cylinder is to pressurize the cylinder at the end of stroke and measure any leakage past the seal. This is commonly referred to as "end-of-stroke bypass test"
    The major limitation of the end-of-stroke bypass test, is it generally doesn't reveal ballooning of the cylinder tube caused by hoop stress as a result of under designed cylinder wall thickness or reduction of wall thickness through excessive honing.
    The ideal way to test for ballooning of the cylinder tube is to conduct a piston-seal bypass test mid-stroke. The major difficulty with doing this is that the force developed by the cylinder has to be mechanically resisted, which in the case of large diameter, high-pressure cylinders is impractical.
    However a mid-stroke bypass test can be conducted hydrostatically using the intensification effect. The necessary circuit along with a step-by-step procedure for conducting this test is shown here

    Yours for better hydraulics knowledge,
    Brendan Casey
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    The next installment from Brendan Casey:

    Hello John,
    In your last few hydraulics bulletins, we've talked about repairing and testing hydraulic cylinders.
    Today I want to explain some of the ways you can increase the service life of your hydraulic cylinders.
    A major cause of reduced service life is damage to the surface of the cylinder rod. Dents and gouges in the rod's hard chrome surface reduces the life of the rod and wiper seals.
    Not only that, it gives dust and other contaminants an easy path into the hydraulic system, increasing the load on the systems filters.
    The potential for damage to cylinder rods and wiper seals is an ever present problem - especially for users of mobile hydraulic equipment.
    One way to minimize this problem is to install a protective shroud or bellows to any cylinders exposed to impact damage. This helps protect the rod's surface from dings and scratches. In abrasive or corrosive environments, it also helps extend rod and wiper seal life and provides an extra barrier to the ingression of contaminants via the cylinder rod.
    That said, the use of a protective shroud is not practical in all cases. So in your next hydraulics bulletin in a few days time, I'll explain an alternative that offers similar life extension benefits.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    Hello John,
    In your last hydraulics bulletin, I explained the benefits of installing a bellows or shroud on a hydraulic cylinder.
    Today I want to talk about alternative rod-surface treatments - to conventional electroplated hard chrome, and the life extension benefits they can offer.
    The first of these is High Velocity Oxygen Fuel (HVOF) which is essentially a metal spraying process. When applied correctly this surface treatment has superior hardness and impact, corrosion and wear resistance to hard chrome. This means HVOF cylinder rods typically last longer in abrasive or corrosive environments than their hard chrome counterparts.
    Black nitride is another alternative. It's an atmospheric furnace treatment developed and patented in the early 1980's. It combines the high surface hardness and corrosion resistance of nitriding with additional corrosion resistance gained by oxidation.
    When it comes to its performance, I've heard mixed reports. One client reports three times the service life in corrosive environments when compared with conventional hard chrome.
    While another client trialled black nitride on salt trucks and found the rods started flaking after one year's service.
    I've also heard one report of long, small-diameter black nitride rods snapping under load.
    A third alternative is nickel under chrome. In the USA, Prince Hydraulics markets this rod treatment as Royal Plate. And this nickel/chrome coating is also offered by Socatri of France.
    Although I've had no direct experience with nickel/chrome plating, according to reports from a reliable source, its corrosion resistance is superior to conventional hard chrome and black nitride - but not as good as hard chrome plated stainless steel.

    Yours for better hydraulics knowledge,
    Brendan Casey
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    Traps! More from Brendan Casey

    John,

    Clause 5.4.6.5.1 a) of ISO 4413 "Hydraulic fluid power - General rules and safety
    requirements for systems and their components" states: "hose assemblies shall be
    constructed from hoses that have not been previously used in operation as part of
    another hose assembly and that fulfil all performance and marking requirements
    given in appropriate standards;"

    This means squeezing a new end onto a hydraulic hose that has previously been in
    service contravenes ISO 4413. In the majority of cases, the practical implications
    of this directive are not burdensome. After all, in most situations, the hose end
    outlasts the hose itself.

    But one situation where this directive can be a 'fly in the ointment' for some
    hydraulics users is in the case of umbilical length hoses. For example, say a 15
    meter length of 2" multi-spiral hose sustains external damage within 5 meters of
    its end, but the rest of the hose is otherwise in serviceable condition. In such
    situations, due to the high cost of the hose, an economical solution may be to cut
    5 meters off the original hose, squeeze a new end onto it and join a new 5 meter
    length to it. You may have seen this done. I know I have. But it's not consistent
    with good practice--or compliance with the Standard.

    Nothing is a problem until it becomes a problem. Meaning in the above scenario, so
    long as the re-terminated hose doesn't blow its new end off, all is good. But if
    the end blows off causing personal injury or significant oil spill, and a
    Standard-waving safety inspector comes around, then it IS a problem. In most
    jurisdictions, Standards such as ISO 4413 are not L-A-W. But they will be
    referenced should a civil claim eventuate. Which means re-terminating a hydraulic
    hose can turn out to be a very costly mistake.



    Yours for better hydraulics knowledge,

    Brendan Casey

    The Industrial Hydraulics Handbook
    Agtronix - the home of the Weedswiper

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    Re: Hydraulics myths

    Quote Originally Posted by john maddock View Post
    Traps! More from Brendan Casey

    John,

    Clause 5.4.6.5.1 a) of ISO 4413 "Hydraulic fluid power - General rules and safety
    requirements for systems and their components" states: "hose assemblies shall be
    constructed from hoses that have not been previously used in operation as part of
    another hose assembly and that fulfil all performance and marking requirements
    given in appropriate standards;"

    This means squeezing a new end onto a hydraulic hose that has previously been in
    service contravenes ISO 4413. In the majority of cases, the practical implications
    of this directive are not burdensome. After all, in most situations, the hose end
    outlasts the hose itself.

    But one situation where this directive can be a 'fly in the ointment' for some
    hydraulics users is in the case of umbilical length hoses. For example, say a 15
    meter length of 2" multi-spiral hose sustains external damage within 5 meters of
    its end, but the rest of the hose is otherwise in serviceable condition. In such
    situations, due to the high cost of the hose, an economical solution may be to cut
    5 meters off the original hose, squeeze a new end onto it and join a new 5 meter
    length to it. You may have seen this done. I know I have. But it's not consistent
    with good practice--or compliance with the Standard.

    Nothing is a problem until it becomes a problem. Meaning in the above scenario, so
    long as the re-terminated hose doesn't blow its new end off, all is good. But if
    the end blows off causing personal injury or significant oil spill, and a
    Standard-waving safety inspector comes around, then it IS a problem. In most
    jurisdictions, Standards such as ISO 4413 are not L-A-W. But they will be
    referenced should a civil claim eventuate. Which means re-terminating a hydraulic
    hose can turn out to be a very costly mistake.



    Yours for better hydraulics knowledge,

    Brendan Casey

    The Industrial Hydraulics Handbook

    Hi John,

    I expect Tasmanian hose assemblers are the same as ours here where crimping new ends on old hoses has become a no-no? Been like that now for maybe 10 years at least now I expect.

    Ironically I steered away from one brand of hose and ends which had the tendency to blow crimps off new hose, let alone used! the design used a particularly short sleeve on the crimp making it compact, but in service especially on 1" hoses such as flail hedge-trimmers it was common for the crimp to separate from the hose just from lack of actual hose crimped to the fitting.
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