If a hydro fails what part of it fails ???

JerryG · Nov 10, 2004

Roughcut said:
Ok.... whattever...I will go with RickB for my info,I think he has probably forgotten more about hydro's then you know 

Billy
 It ain't no gear vs hydro thread.
I know that. That is why I don't understand why roughcut said the above to Slowery. O'well lets just drop it before it goes in the wrong direction any farther than it has.

JerryG · Nov 10, 2004

"Something inside..."
I'll go for that!!!!! /forums/images/graemlins/cool.gif

RobS · Nov 10, 2004

Yet another way to look at it "what are the likely failure modes of a hydro?" Using the gear as an analogy the likely failure modes are probably worn clutch disc, weakened pressure plate springs and worn throwout bearing.

Not knowing much about the internal workings of a hydro can anyone speculate? If it does overheat, what happens? Do seals go south? Parts warp? Contamination plug orifices?

I think this is a very legitimate question regardless of the failure rate. So what are the higher probability failure modes of a hydro?

Quick engineering lesson here: a common tool in use is an FMEA (Failure Mode and Effects Analysis). You list all the possible failure modes then rate them for severity, probability of occurance and likelihood of detection. The product of these three is the Risk Priority Number. Engineers then work hard via testing and redesign to reduce the overall RPN. The field experience with hydro transmissions suggest the engineers have done a decent job at this. Still, there will be some failure modes with higher probablility of occurance. Depending on severity, this is where you may see some belt and suspenders approach to the design.

So back to the question...

/forums/images/graemlins/smile.gif

gsganzer · Nov 10, 2004

Quick engineering lesson here: a common tool in use is an FMEA (Failure Mode and Effects Analysis). 

We use the same sort of thing in my industry called HAZOP review. If a valve fails to open or close, how many people do we kill, bodies of water get poisoned and square miles of homes to evacuate? /forums/images/graemlins/confused.gif

eee · Nov 10, 2004

Thanks Robs/Rick/BillyP.

duane · Nov 11, 2004

Let me take a shot at this question, if a hydro does fail, what is the most common failure mode.

As mentioned, heat and contamination are detrimental to the life of a hydro transmission. A hydraulic pump and motor, aka hydro tranny, are products with precision made internals. Maintaining the close clearances within the rotating components, pistons, vanes, etc, are vital to developing hydraulic pressure and then using the hydraulic pressure to develop a mechanical output torque. Contamination will start to open up these tight clearances, as well as heat due to hydraulic fluid break down and micro boiling which result in metal to metal contact. Once the clearances open up, the ability to develop hydraulic pressure in the pump, and to utilize the pressure in the motor are compromised. A loss of torque results, and is especially noticeable when the tranny gets upto operating temperature due to thinning of the oil. Using a higher viscosity oil will help for a while.

Just my thoughts, I have no side in the debate of hydro or gear, I have and love both.

RobS · Nov 11, 2004

Thanks Duane, now we're getting somewhere. Again, I know nothing of the construction details of a hydro so here are some more questions. Are there rubber seals in the pumping elements or is the sealing by "fit"? There must be shaft seals somewhere in the system. What pressure does the system operate at? Is the pump a piston pump or vane or ???

It's probably been covered before but does anyone have a good schematic and/or cross section of a hydro trans to post?

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kensfarm · Nov 11, 2004

My guess would be the hydraulic pump.. on my farm neighbor's skidsteer.. he's had to have one of the hydraulic pumps rebuilt twice. He's not much into maintenance.. maybe the hydraulic fluid level.. or no routine filter changes have gotten the best of the equipment.

Soundguy · Nov 11, 2004

Here's my take on it. and keep in mind that this comes from a ger tranny guy that has running tractors back from the 40's with no tranny rebuilds... )

I would 'think' that for a well maintained system... either gear or hydro.. the 'soft' parts or the wear/replaceable items will be the major areas of failure. Like shaft input seals on the gear tranny, and the clutch on the gear tranny.

For the hydro tranny that would be orings and seals. I'm speculating that a hydro tranny is pretty bulletproof.. but that a 2 cent oring can make you tear it down, in the same way that a 3 dollar tranny input shaft seal is going to make you split a gear tractor. I know we aren't compairing the two.. but I would look for leaks/seals/orings.. etc to be the major failures of the hydro, not counting actual hard part failure.

To go along with that.. i think heat and foriegn contaminants in the oil will be the biggest contribution of 'soft' part failure...

As someone else pointed out about gear trannies not coming in for worlk.. I agree with the main answer given.. most people are fine about splitting their gear tranny tractor and throwing a clutch in, while parked inthe barn.. while many of us may NOT be ok with tearing down a hydro tranny to look for a pinched oring.. etc.

Clutches on a gear tranny are a wear item.. expect them to go bad.. just part of the 'maintenance' on the gear tranny.. etc.

Soundguy

jinman · Nov 11, 2004

( Is the pump a piston pump or vane or ??? )

Rob, I can tell you a little about NH hydros because I've looked at the diagrams on mine a lot. As I remember, there are two pumps and one motor. One of the pumps is a charge pump that draws fluid from the reservoir and supplies it at low pressure to the main pump.

The main pump is a piston type pump. The wobble plate that controls pump displacement is attached to the hydro pedal(s). The amount of fluid pumped is proportional to the deflection of the pedal. The piston type pump lends itself very well to this application. New Holland's TC35/40/45 pump is capable of producing 5000 psi before relief valves lift and return fluid back to the reservoir.

The hydraulic motor on New Hollands is also a piston-type motor. In these motors, the wobble plate normally has a fixed angle, but this is not the case for the Class III NHs. On the TC35/40/45 transmissions, the wobble plate can be placed at either a 9° or 15° displacement. An electric solenoid selects the displacement by the action of a switch located on the FEL joystick or on a fender mounted switch. The 15° displacement takes much more fluid to operate the motor, so it is the low range, and the 9° position provides the high range. Flipping the switch while moving will give you a dramatic and instantaneous shift in speed.

To my knowledge, NH is the only manufacturer that has the 2-speed motor in it's hydrostatic tranny. I'm not saying others don't have it, but I'm just not aware of it. Both pumps, the motor, and the input/output drivelines fit into a sealed unit. The "transmission casing" around the transmission is just a shell. The output of the hydro tranny feeds into a mechanical two-speed gearbox. Thus, on the NH Class IIIs, you end up with four forward and reverse ranges (two in the hydro tranny and two in the mechanical tranny).

I'd love to hear how JD or Kubota trannies work if anyone wants to give a brief description. /forums/images/graemlins/cool.gif