Hydrostat issue...with video
 

I have a hydrostat issue I'm hoping someone will be able to diagnose. I've posted a video for you to look at, link below.

I think I need another hydrostat, but I'd like to know what is wrong with this one if anyone has any idea. I've disassembled it three times, and can find no issues other than the motor shaft has about 1/8 inch of play in/out. My thought was the motor block was not splined to the motor shaft and at some point (in the video) the splines catch. But the axial play in the motor shaft doesn't seem like it's enough to disengage the splines from the motor block.

It used to work (that is, this axle used to be driven by this hydro), but I suspect that this issue predates my ownership and perhaps has gotten worse over time to the point it no longer works at all.

I've worked on these hydros several times in the past, and successfully rebuilt/repaired a couple. I have the manual and am quite careful about keeping the valve plates, charge pump housing, etc. in correct orientation. The motor block springs are intact, the pistons, slippers, swash plates all look good and the pump and motor shafts are fine.

Here's the video

https://youtu.be/5HAjC0O2wD4

I had one that wouldn't do anything either. put in another one, worked great. took it apart and couldn't find anything wrong.

It looks like the charge pump is moving oil.

And you obviously had the pump fully swashed, forward and reverse?

I'd work backwards from the pinion. How is it coupled to the motor output shaft?

Did you remove the spring pins in the pump swash plate to check that they're not sheared?

I've often seen a lens plate that did not make full contact with the head. It only has to be off a few thousands, usually because a locating pin was holding it up. Some Sundstrand's had two dowel pins, one being round and one having flat sides to go in a slot.

Pump was fully swashed in forward, as it tends to do when not restrained.

The pinion is part of the motor shaft--its all one shaft.

did not remove the pins in the pump swash plate, but verified not sheared by applying some force to the trunnion arm in both directions. Seemed OK.

The lens plates (manual calls them valve plates) key to pins in the cover. They are marked differently so you can tell the pump from the motor plate. I install them with a dab of grease to hold them to the cover ("central section" I think they use in the book--the charge pump cover is the front section)

Just another bit of explanation--this hydro started this behavior before I took it apart. It worked OK for a while, then developed this problem. I only took it apart after it started behaving this way, so it is not something I did.

Still trying to determine what is causing this sudden violent impact. There is no damage to the splines, either inside the piston blocks or on the pump and motor shafts, no metal particles evident inside the unit.

That's my plan at this point. I had one that was destroyed by towing, (cracked the housing) and another that was slightly damaged by towing. I built one good unit from the two and it's been running fine for 4 years now.

So the pinion gear and motor shaft are machined from a billet and not two mated pieces?

The motor is fixed displacement. The pump is the only thing that can go to neutral displacement and you're convinced it is swashing.

Is there spring pressure on both blocks when you put the cover back on? When you loosen all the cover bolts, the cover should lift off the case slightly.

The manual shows the motor shaft as a single part, but I did not remove it to inspect it to be sure it's not assembled from two parts somehow. It feels like a single piece--that is, the pinion is not loose on the end of the motor shaft.

I can feel some resistance in the trunnion arm with the drill driving the hydro. The drill speeds up when the trunnion is in "neutral" so I believe it is swashing the pump.

There is spring pressure on both blocks, yes the cover springs up about 3/16 inch. I did not remove the springs from the piston blocks but I looked at them carefully with a flashlight and they appeared in good shape.

Sometimes when new bearings get installed in the cover plate, they're not seated quite deep enough and that holds the lens/valve plate off the cover just a bit, same for locator pins. Or there is a burr or bump on the cover or lens plate.

Just to eliminate some simple things, swap in relief valves from a working unit. Just the charge pump going over relief should haul your drill down.

Since you have to pull the hydro anyway, might as well open it up one more time. I've rebuilt hundreds of them in 28 years in the shop, but never "online".
Might be fun to do some virtual hydraulic sleuthing.

And I believe you're correct about the pinion.

What oil are you using ?? :Huh:

As far as the belt destruction: is it possible the rear trans returned to neutral which would essentially force your front trans pull it, putting a heavy sudden load on the belt?

Its also possible the two issues are unrelated and coincidental.

And a tip for anybody posting vids from a phone, hold the phone sideways. :beerchug: