JD 70A lift speed issue

[Mechanos]

Ok... that is what is happening with mine. I can raise the boom (to any height) and leave it there. Come back hours later and it there is no noticable drop in the boom height.

So help me wrap my head around this.... if the piston seals are that worn, what keeps the oil from bypassing around the seal and letting the boom creep down? Is it the differential volume between the two sides of the piston? In order for the piston to move (downward) it would have to displace a greater volume of oil on the side without the rod than the "newly created" volume on the side with the rod. Since the SCV is holding tight, this is not possible so the boom stays put?

 

[J_J]

Volume is what give a hyd system the speed. Are you sure your system is pumping the specified GPM's.

If your lift arms stay up over 24 hr, then you can assume that the cyl's and valve are good.

Someone may have changed out your hyd pump for a smaller GPM version, or you are diverting some of that fluid to other parts of the tractor, maybe power steering, etc.

Do you have a hyd flow chart.

Next thing would be to put a flow meter on it and measure the flow in GPM's.

 

[J_J]

Back to the fitting again if everything else is good. Did you eyeball each fitting in the lift circuit. Some people want a slow loader when they first use a loader, and forget later on, that the restrictors are still in the circuit. QD's will slow the flow a little. Are the QD's allowing the full flow to the cyl? If your pump flow is 5 GPM, do you think you can fill a 5 gal bucket in one minute? Try this with the hose removed off the cyl port. Keep in mind that the GPM's is flowing to two cyl in normal operation.

If the pump is putting out a certain GPM, and you know the cyl dimensions, then the speed of the cyl operation can be computed. The mechanical hook up may degrade your times.

 

[Mechanos]

Volume is what give a hyd system the speed. Are you sure your system is pumping the specified GPM's.

If your lift arms stay up over 24 hr, then you can assume that the cyl's and valve are good.

Someone may have changed out your hyd pump for a smaller GPM version, or you are diverting some of that fluid to other parts of the tractor, maybe power steering, etc.

Do you have a hyd flow chart.

Next thing would be to put a flow meter on it and measure the flow in GPM's.

The pump is the correct pump my tractor. Rated at 7.2 GPM. Flow is divided between the hydraulics and power steering. 70%/30%... so that's 5 GPM through the hydraulics.

Here is the hydraulic schematic... mine is the 955.
View attachment Hyd schematic.pdf

Back to the fitting again if everything else is good. Did you eyeball each fitting in the lift circuit. Some people want a slow loader when they first use a loader, and forget later on, that the restrictors are still in the circuit. QD's will slow the flow a little. Are the QD's allowing the full flow to the cyl? If your pump flow is 5 GPM, do you think you can fill a 5 gal bucket in one minute? Try this with the hose removed off the cyl port. Keep in mind that the GPM's is flowing to two cyl in normal operation.

If the pump is putting out a certain GPM, and you know the cyl dimensions, then the speed of the cyl operation can be computed. The mechanical hook up may degrade your times.

My tractor's reservoir is about 4.5 gallons, so I'd have to time the discharge of a smaller volume of oil and extrapolate from there.

Here's how I tapped into the system for a pressure gauge. I removed a 90° fitting at the pump and replaced it with a capped Tee. I removed the cap and attached my pressure gauge to the Tee.


Here are the results:

Rockshaft relief: 2750 psi
Bucket curl relief: 2750 psi
Bucket dump relief: 2750 psi
Boom lower relief: 2750 psi
Boom raise relief: ~2550 psi average... very irrattic readings... needle bouncing all over the place.​

Based on the readings, it looks like I have bad piston seals in the boom cylinders as suggested by others. I did not try to identify which cylinder as I'd replace the seals in both to keep them matched.

 

[Mechanos]

I've got a couple of seal kits on order... hopefully they'll be here by the time the shop closes on Saturday. Kind of shocked at what a seal kit cost from the dealer... $43/each.

Tonight I'm going to try to measure the flow from the pump. I'm thinking I'll just the time the discharge and see how long it takes to fill a 1/2 or 1 gallon container. With only a 4.5 gallon reservoir, I don't want to pump too much fluid out.

 

[J_J]

The hyd flow will depend on the rpm of the engine. if direct coupled .

 

[Mechanos]

Yes. It's direct coupled. If I remember right, the pump data plate states the rated flow at a certain RPM. I didn't get a chance to fool with it tonight... ended up working late.

Just looked in my tech manual and the only flow specs they give for the pump prior to any device... total pump flow. Books says 7 GPM at 3450 RPM. If I check the flow out of a boom cylinder hose, I'll guess I'll just have to take 70% of the pumps rated flow as that is what the flow divider is supposed to be supplying. So, that should be 4.9 GPM at 3450 RPM.

 

[dfkrug]

So help me wrap my head around this.... if the piston seals are that worn, what keeps the oil from bypassing around the seal and letting the boom creep down? Is it the differential volume between the two sides of the piston?

Yeah, sort of. But it is simpler than that. You are trying to force a
piston/rod into a sealed cylinder full of an uncromprssible fluid.

To make the thought problem easier, let's say you can eliminate the
hoses, fittings, and valve by just capping off the ports on your cylinder.
Now imagine a blown piston seal, so oil is free to get around the piston.
Now if you make the blown seal path bigger (like no seal at all, or no
piston at all) that should make no difference on whether or not the
cylinder can contract, correct? If you are trying to push a rod into a
sealed cylinder filled with an uncompressible fluid, you will increase the
pressure on the fluid, but nothing will move.

 

[J_J]

What if there is a pressure differential caused by the weight of the bucket. Will the rod extend, and the rod side fluid transfer to the base end?

 

[dfkrug]

What if there is a pressure differential caused by the weight of the bucket. Will the rod extend, and the rod side fluid transfer to the base end?

If the piston seal leaks, there is no pressure differential between the
fluid on each side of the piston. It is a basic tenet of hydraulics that
any pressure on a vessel of a non-compressible fluid will be experienced
throughout the fluid (Pascal's Law). A leaky piston seal, (or no piston
at all) turns all internal cavities of the cylinder into one connected vessel.