Hyd valve and cyl leakage rate.

[J_J]

If a hyd valve leakage rate around 10 drops per min is OK, how many drops per min would be allowed on a cyl to qualify as normal or bad.

Consider this, that if a single cyl were to transfer a quart, or 32 oz, or 18,968 drops of hyd fluid in 24 hr, from one side of the piston to the other side, would that be considered normal, or the need to replace seals.

That equates to 13.172 drops per min,

So, how do you/they rate cylinder leakage as to exceptional, normal, bad.

The over all time for a complete leak down rate on a small bore cyl would be more apparent than a larger cyl bore, because of volume.

Would weight on the cyl be a factor on the leakage rate?

 

[Wayne County Hose]

Most of the factory leakdown rates I have seen published are with the cylinder at full rated capacity. I don't know how they figure how much leakage is acceptable.

 

[DJNebraska]

Interesting question JJ. :thumbsup:
I don't know who can really answer it (or want to). I doubt manufacturers of hydraulic cylinders would put in writing or even talk about leakage. I'm my experience with hydraulic cylinders, those with piston lip-seals are nearly bubble-tight and will hold a load for quite some time. Those with piston o-ring seals tend to leak more.

While the cylinder manufacturers probably don't talk about it much. Those who build machines may address the issue, like Toro does here...

http://www.greensdepartment.ca/files/Download/00-05.pdf

 

[J_J]

I would think the logic would be that if you can make the relief valve come on, that the cyl is normal and can do the job.

That is not to say that if the bucket is full and you are transporting the load, if you keep having to adjust the cyl level, that if it becomes a nuisance, one should have the cyl seals replaced.

Similar to the 3pt, if the cyl can not keep the attachment in place, then the seals need some work. Some times the valve and/or the relief valve is at fault, but usually it's the piston seals.

 

[oldnslo]

JJ,
The only time I have ever seen any leakage specs on a cylinder was for special applications like DJ pointed out.

I agree with the use for FEL where if the load is dropping to fast you will need to determine the leak source and repair it.

If leak down of an FEL or implement is an issue companies do make valves with built in load holding checks. I believe some have hydraulically pilot operated checks and others use mechanically opened checks. These are usually used on higher end equipment and not on CUT's or SCUT's.

Roy

 

[techman]

Cylinder leakage rates are rarely published. The rates are primarily a function of the types of seals. Seals that have more leakage generally have lower friction, that is critical in some applications. For example, iron ring seals have very low friction, but high leakage. At the other end, "U" cup seals have virtually no leakage, but very high friction. In the middle are step seals, which are the most common.

If leakage is a factor, then you can specify seals that will suit that requirement. Generally load will relate to internal pressure, which relates directly to leakage.

Generally cylinder drift is due to the leakage in the valve connected to the cylinder. Near leak free valves are available, but not generally used on lower end hydraulics. Many cartridge type counterbalance and relief valves are little to no leakage, but spool directional valves are "leaky". Leak free directionals, using poppet valve technology, are available, but at high price.

paul

 

[J_J]

In my observations, concerning cyl leakage, When ever someone complains about the loader not staying up, most of the repairs have been done to the cyl to fix the problems. Very few have said that that was not the problem, and they had to fix the valve. The ratio of cyl to valve replacement is probably around 10 to 1. I am mainly talking about internal leakage. Some outside leakage can be fixed by replacing o-rings on the spools.

Valve and cyl leakage doesn't seem to bother some people, they keep adjusting the loader to get the job done. and they sometimes wonder why they have to keep adjusting the lift arms. Same thing with the 3pt, it should not keep falling with the lever in neutral. Lift arms descending over night seems to be standard, or a lot of them do it.


In my computations above, it would take 31 hrs to leak a quart of fluid at 10 drops per min. So if 10 drops is permissible, then if the loader is down in 24 to 31 hrs, that is good enough for the manufacturers, and I guess we should not worry about it. If you can see the lift arms descend, that is a problem, whether it is caused by the cyl or the valve.

Like I stated before, if you think the valve is bad, or worn out, it can be tested with a hyd hand pump and gage. and you can see the drops coming out the valve. There are different reasons for a valve to leak slow or fast, and that depends if there is any pressure on the spools.

Would it not be possible to pressurize the valve gallery, and plug the PB and OUT port, and put about 500 psi in the valve gallery and see if it leaks to any of the work ports. Could do this in reverse, and pressurize the work port and see if the spools leak.

You guys that do hyd for a living, how do you test spools and cyl? Do you all do leakage test, and produce a report for the customer, or just tell them it needs replacing.

 

[techman]

I
You guys that do hyd for a living, how do you test spools and cyl? Do you all do leakage test, and produce a report for the customer, or just tell them it needs replacing.

Doing a simple cylinder test is easy. Extend (or retract depending on ports) the cylinder. Remove one hose and cap it. Put a hose on the open cylinder port and put the other end of the hose in a bucket (or similar). Pressurize the cylinder with the still connected port. Check for leakage flow in the bucket. It is advisable to duplicate the test with the opposite cylinder port since seals will occasionally seal in 1 direction and leak in the opposite.

Valve leakage methods can vary. For a blocked center spool, pressurize the P port and look for leakage in the service ports. If it is an open center (power beyond) you need to block the T port and pressurize the P port and check the service ports for leakage.

In production testing, the production test stand does all of this for you, with a low flow flow meter measuring the leakage flow. Total leakage on a 1/4" industrial valve is 25 cc/min. This is from P to all ports in a closed center valve.

paul

 

[IceStationZebra]

The only industry that I know has a specified leak down limit is industrial fork lifts in Europe. It can be a tough spec to meet, and is complicated by the variance in oil viscosity. But the cylinder wasn't the problem, it was the valve. We had to limit the valve spool clearance to ~0.0005" diametric clearance, which required the vendor to select fit the spool to the bore. The tight fit also required that the customer service the valve section and spool together, otherwise we couldn't guaranty the fit.

Otherwise, it basically comes down to what is acceptable during normal operation and safe.

ISZ

 

[oldnslo]

Friend of mine worked on the control valves for Case back Hoes. They had a very tight leakage spec for their valves and it was also controlled by matching the spools to the bores. like techman said, this was all done on test bench in very controlled environment. Take that same valve and operate it at 40 degree F and 140 degrees F and you will get two totally different leakage rates.