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# Splined Shaft Coupling Issues8

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## Splined Shaft Coupling Issues

(OP)
I am having repeat failures of a splined coupling. The system consists of belts and a pulley driving a splined input hydraulic pump through an adapter that is flanged on one end for the pulley and female splined on the opposite for the pump. The splines keep fretting away to nothing. The system came packed with a moly paste type lubricant that quickly gets hammered out of the connection. We switched to an EP grease which quickly gets centrifuged into base oil and thickener. We tried a gear coupling grease, Kop-Flex KSG, which was too thin and passed through the coupling too quickly. There are no seals. We have settled on Mobilgrease 28 which has enough viscosity to stay in the coupling and is stabil enough to make it to a reasonable lubrication interval. We are still getting some random failures here and there, though.

One thing I have noticed is that the female spline is not hardened. A center punch leaves an indentation similar to A36 steel. We are thinking about making our own adapters and matching the hardness of the pump shaft.

However, I have seen that it might be possible to use an anaerobic adhesive to make the coupling rigid and eliminate fretting corrosion. I am interested in pursuing this route and just wanted to see if there was any experience with it here.

### RE: Splined Shaft Coupling Issues

2
Is this a power take-off from a diesel engine? If yes, then you may have a torsional resonance.

Walt

(OP)

### RE: Splined Shaft Coupling Issues

Hi,
i don't know how much you are involved in the design of that splined connection. Also, i don't see the overall picture of the drive train, so assuming from some keywords (pulley, fretting, hammering) imo there's some design checking to be done, notably:
- Were radial force loads correctly considered as well as the pure torque loading?
- Did design decisions consider momentary overload situations, shorttime peaks, sudden load changes (if applicable), change of load direction?
- Are centering, alignment, tooth pressure within spec. / design standard requ.-mts?

These spline connections are quite a good loadtransmitting element, if male/female come together w/o play ("centering"), if axial alignment of male/female is within spec. and if load distribution allows a sufficient number of teeth to share the load.
Lube should not be the issue, imo the moly paste originally applied was to facilitate disassembly. Male / female should not move, let alone fret constantly.
Material might be a part of the case, that again is a design issue.

Regards

Roland Heilmann
Lpz FRG

### RE: Splined Shaft Coupling Issues

Hi

My ten penny worth, find out why the are failing first rather than experimenting with solutions with unknown results its cheaper in the long run.

What torque and or is the coupling transmitting? Quite recently I went to look at some screw jacks that kept failing so I did some basic calculations based on information from the manufacturer and found they were overloading the jacks beyond the recommended rating.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Splined Shaft Coupling Issues

(OP)
No, I had nothing to do with the design process, I am an end user. We have come up with solutions for most of the little problems we see on these drives but this one keeps coming back. The manufacturer appears to have changed from moly paste to some kind of grease for lube but that is all. I was trying to avoid spilling the beans on what the equipment is but..

*pictures removed to protect identity of manufacturer*

### RE: Splined Shaft Coupling Issues

2
There are many types of splines, can you easily remove the spline hub for service or is heat required?

The main advantage of a spline is that is transmits torque very efficiently and self centers the transmitting components yet allows for easy removal and service. The only real downside is that they are costly to manufacture.

Where traditional clearance fit splines are not good is when you put a belt on it, to drive torque via a side loading condition. This mitigates the primary advantage of the spline, its self centering ability. It also creates a binding wear condition where the spline will continue to grind away on itself because the male and female components are off center due to the side load of the belt and pulley system.

What you need a major diameter fit spline, or and interference fit hub. Unfortunately this will render its other primary advantage useless, its "easy removal."

A compromise would be a taper fit hub, which can give you some easy service ability but not as easy as a clearance fit spline.

What makes the problem even worse is that you are dealing with a vibratory torque input, looks like you are dealing with a slight design flaw, but I am sure it can be corrected.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

(OP)
There is a driver which is supported by two deep groove ball bearings between the pump shaft and pulley.

### RE: Splined Shaft Coupling Issues

If I am understanding what I am seeing, the manufacturer has done an amazing job restricting the movement of the splined connection, being that the hub is supported by 2 generous bearings. But a clearance fit splined connection should be allowed to find "home". With this current set-up that will require the run-out of that spline to the bearing surface to be perfectly controlled, even a few thousanths run-out will grind the spline away rapidly, any ideas on how to check that spline run-out to those bearings?

On a side note, I think they already knew they had a problem, it seems highly over-designed.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

I see the two radial bearings that carry the radial load from the belt, so I can't see a change in spline form as helpful.
I'm assuming that the olive drab colored adapter shell is fairly stiff, and equipped with a pilot for the hydraulic pump, so misalignment or mispositioning of the shaft vs the hub is likely not a problem.

Which leaves hardening of the female splines. I doubt that an experienced manufacturer would forget to do that, though a manufacturer's Supply Chain Manager might eliminate the process step in order to 'save time'.

I suggest you have some hubs made with hardened internal splines, or find a coupling hub with hardened splines and get a machinist to join the splines from the hard hub with the remainder of the OEM hub, with Dutch pins or other keying scheme.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Splined Shaft Coupling Issues

As usual Mike is making some great points, only concern with that solution is that if there is a design induced run-out problem, then that could just transfer the problem to the bearings.
When it comes to spline failures, typically the hardest surface wins. I caution the conclusion that it is a hardening problem, maybe on a one or two case basis, but continually? either way it is easy to rule out, so I agree with Mike just verify that if you haven't already.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

(OP)
We're willing to make a whole new driver. Is there any preferred steel for female splines? 1040 or something with some alloy?

I will see about getting the driver hardness tested. In 10 years we have gone though close to 10 of these. They're all soft when punched with a center punch. Would there ever be a reason to make a spline soft?

For the running clearance, the pump does fit the green colored housing with a register. Some of the bearings have spun in the housing resulting wear to the housing. The wear becomes obvious as it causes uneven tension of the 6 drive belts. The housing wear is on the magnitude of 0.015mm by the time we see changes in belt alignment. The nominal size is 130mm. We have not seen any change in spline wear rate in the worn housings. I am having the worn housings hard chromed to restore the ID. I have specified an H7 fit for the repaired housing.

### RE: Splined Shaft Coupling Issues

Typically a splines function is not to be like a gear, there should be very little movement in the spline when it is under torque, so in most cases when we have seen spline wear it is a symptom of another problem. If you are looking for it to withstand wear, I would recommend an 8620 steel with a carburized spline. Should be able to get that up to RC60 or higher locally at the wear surface. But you may end up just wearing out the external spline component.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

(OP)
Just a little more information, the shaft spline appears to be DIN 5480 40x2x30x18x9g.
The maximum power the unit can draw is about 150 horsepower at 2400 RPM. That is at 3000 psi but the system rarely operates at over 1000 psi.

### RE: Splined Shaft Coupling Issues

I like the idea of taking the slack out of the spline with Loctite, but that will cause bearing problems unless all the parts are made really well, because then you end up with a short composite shaft having at least three, maybe four, bearings supporting it.

OR unless you provide some lateral flexibility in the composite shaft, e.g. by:
- Making the female spline part of a shaft that extends completely through the bearing-supported sleeve past the timing belt pulley, where the pulley/sleeve and shaft are joined by a radially flexible coupling, or
- the female spline ends in a severely necked down round bar that ends in a flange bolted to the sleeve end, like a full floating truck drive axle. It would all work better if there's quite a bit of axial space on the non-pump end of the drive so that necked down (and spring tempered) shaft can be long enough to allow some misalignment/offset of the various bearings involved.

IF the parts are pretty good, you could just assemble parts to the existing design with some Loctite in the splines, and let the Loctite cure _while_ the drive is running at low load.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Splined Shaft Coupling Issues

Too bad the OP removed the installation photos so as to not embarrass the guilty.

Working from memory, I'm thinking that if the pump were moved 'aft' a couple of feet, it could be driven by a slightly flexible shaft with male/female splined ends, both ends secured to existing parts with Loctite in the splines.

... at some expense in weight of bracketry, possibly not in cost if you think of the pump/bearing assembly like a pivoting jackshaft and get rid of the machined slide assembly.

Mike Halloran
Pembroke Pines, FL, USA

### RE: Splined Shaft Coupling Issues

I would lean toward a torsional resonance, but that is hard to discuss and prove in a forum with limited information.

Walt

### RE: Splined Shaft Coupling Issues

(OP)
I read the Loctite bit in an old Danfoss article. I think this is a good candidate for it because the two deep groove ball bearings on the driver have a radial I clearance fit in the housing of about 0.0005-0-0015" and about 0.025" of axial clearance. The pump shaft has one cylindrical roller bearing and one plain journal bearing. There is some axial freedom there but it's hard to measure because the shaft is driven forward by a spring. It's at least 0.010"

### RE: Splined Shaft Coupling Issues

(OP)
Walt, the belts do vibrate up and down a bit on the loaded side. This could be a sign of some resonance? I will try to make and post a video soon.

### RE: Splined Shaft Coupling Issues

I am with Strong, you definitely have a vibratory torque condition, but I don't see how you can eliminate it being that the drive train is set. I go back to the fit on the spline hub, clearance or run-out variance will cause the spline to just grind itself away under vibratory torque conditions.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

Well, sure you have vibratory torque with a Diesel prime mover, even absent resonance.
Maybe some compliance would help.
For instance:
A small accumulator on the pump discharge.
A spring loaded idler on the drive belt, on the driving side.
A slightly flexible, hollow, toothed idler sprocket, inserted between the belt runs, free floating, i.e. with no bearing or pin or fixed support. (This may have been patented; I don't know if it has been commercialized; I haven't seen it much in the wild. It would probably need flanges to keep it from riding out of the belt, and the flanges would have to be segmental to maintain the flexibility of the sprocket.)

Mike Halloran
Pembroke Pines, FL, USA

### RE: Splined Shaft Coupling Issues

(OP)
It's a variable displacement axial piston pump, Rexroth A4V 125 displacement code, with an additional gear pump ganged on the back. The gear pump is probably around 10hp consistently. The variable displacement pump spends most time in the 0 flow position and has a soft start and stop by pulsing the servo piston solenoids. It's operating a slewing gear.

### RE: Splined Shaft Coupling Issues

Does The OEM "grease" solution happen to be one of the Kluber assembly pastes? One of those was a pretty effective solution on a machine tool spline situation I'm aware of involving hundreds of precision spindles a few decades back. Those splines were very precisely manufactured and located.

Some of the pictures appear rather dry and lots of red fretting debris is present. I wonder if weekly re-lubrication is possible, or an oil bath or even total-loss oil lube could be arranged.

I know one experienced spline person on this board maintains there are situations when lubrication is to be avoided.

### RE: Splined Shaft Coupling Issues

You mentioned that the pump runs most often at low/no load. A conventional gear drive operating at low load with a torsional resonance present can cause the gears to chatter. If this is the case with the spline shaft, then I doubt lubrication change would be the answer to the problem. In the case of a diesel powered fishing vessel with two brand-new engines and and transmissions, I recommended a flywheel be installed on the problem starboard drive engine that solved the vibration problem. A good set of vibration measurements possibly including torsional vibrations and sound pressure (for belt vibrations) would really help get to the root cause of the problem.

Walt

### RE: Splined Shaft Coupling Issues

It think it was mentioned that it is spinning at 2400 rpm in a previously deleted post. That lends me to believe that any type of grease will be ineffective. At those speeds grease just is centripetally forced away from the contact surfaces, especially since the fit allows for clearance.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

My opinion the male and female spline are of in adequate design, based on the type of material and the type of heat treat. the male spline should be crown lead to allow for misalignment. both splines should be case harden premium steels and coated with drylube. the splines should be similar to ANSI b92.1 class 4 splines. there should be adequate process control with manufacturing and heat treating.

### RE: Splined Shaft Coupling Issues

in addition if better alignment is required a major diameter fit spline, in addition with a crown major diameter on the male spline.

### RE: Splined Shaft Coupling Issues

(OP)

I thought I added it but the pump shaft is a DIN 5480 40x2x30x18x9g.

### RE: Splined Shaft Coupling Issues

(OP)
Ok, got the pieces hardness tested. The driver is 29 HRC, possibly 4140, and the pump shaft is 50 HRC.

### RE: Splined Shaft Coupling Issues

If it is 4140, then you could just take the replacement part and have it hardened to 40-42 HRC. But I definitely think going about 45 HRC could expose your pump shaft to significant wear.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

### RE: Splined Shaft Coupling Issues

(OP)
The heat treatment shop recommended going to 40 HRC and broaching after heat treatment. Any higher and the machine shops won't touch it.

### RE: Splined Shaft Coupling Issues

hmmmm I still believe the surfaces need to be case harden for wear, if AISI 9310 is uses the parts can be normalized and harden while in the bar stock condition to 27-36 HRc which perfect for machining and broaching.
then carburized and harden with a .020-.025 effective case depth, core hardness of 33-43 HRc, then lap with cast iron. to obtain size. & od grind any close tolerance diameters.
the surface will be wear resistance (Hertz) and the core will allow for over torque or vibration. or the desired yield and ultimate tensile.
or use 300M or 4340M same as above but instead harden to 50 HRc by vacuum gas 2 or more Bar quench. the problem with soft splines is excessive wear. then it self destructs.
300M will not distort with a high bar quench.
Important!!!!
the spline P.D.'s must be concentric to the registered diameters within .0015 TIR max.

### RE: Splined Shaft Coupling Issues

(OP)
Under $1100 is my target cost for each driver. Lapping sounds expensive? Grinding is expensive out here, too. It's$600 for the bore of the driver housing.

### RE: Splined Shaft Coupling Issues

send quotes to gear shops and see what the cost will be. at that price,it is why there are failures. cheap parts.
add the cost of down time plus parts and labor, I bet it's more than \$1100 . just saying. a spline lapping operation is a simple and quick operation if broach correctly.

though harden parts is the most cost effective method but does not wear as long. I would highly recommend a crowned spline. it is simple to manufacture.
and coat it with dry film lube.

### RE: Splined Shaft Coupling Issues

(OP)
I did find a local gear shop so the RFQ is in.

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