[AT] hydraulics question

[Tom]

Dudley Rupert wrote:
> 
> Cecil,
> 
> I am not a hydraulics expert but I have taken three maybe four two-way
> hydraulic valves apart in the past two or three years, cleaned them up,
> honed the cylinder bores, polished the pistons and installed new O-rings.
> I've installed two of these valves on two different tractors to use for
> controlling aftermarket 3-pt hitches and when these valves are in the
> center/neutral position they will hold a load for days with no noticeable
> compression of the hydraulic cylinders/lowering of the hitch.
>............. 
> The bottom line, as your' manual indicates, is one valve failure could be
> the cause of both cylinders sagging an equal amount.  But, you've replaced
> the valve so let's rule it out for the moment.  If there are no external
> leaks between the valve and cylinders then it seems the seals in both
> cylinders have failed.  If the seals in only one cylinder had failed then I
> would think that the other cylinder could hold the loader up by itself
> particularly if the bucket were empty.  I assume you've tried this and that
> the loader will not stay up even when empty so we're left with two cylinders
> to rebuild.
> 
> .............
> 
> Dudley
> Snohomish, Washington
> 
 Dudley
	I've got to take issue with a couple of your statements above.
	A hydraulic control valve relies on the fine clearances achieved
	at manufacture to perform. To hone a valve body is going to only
	increase internal leakage unless one is in a position to fit 
	oversized spools. O rings are fitted to prevent low pressure
	external leakage as there is generally a gallery back to the return
	inboard of the O rings.

	Secondly, oil supply to the cylinders is common, therefore if one
	leaks internally the drop in load gets shared between them.
	Otherwise, to maintain equality in stroke length, there	have to be
	flow dividers in their supply circuits to ensure balanced flow to the 
	cylinders.

 Cecil
	With the problem at hand, water contamination most probably has
	contributed to the deterioration of the seals. If the original type
	seals aren't available, there are quite a few manufactures that can
	supply synthetic seals of more modern design that will either fit
	in the original locations or can be fitted with the remachining of
	the pistons. I've found it hard to go past Greene Tweed seals in
	when rebuilding & modifying cylinders.

 Tom