Grease For Rotating Coupling

[TugboatEng]

I have a rotating splined coupling that I am trying to determine the best grease for. High shear resistance is of greatest importance as all tested soap thickened greases separate too quickly in service. I also need a NGLI 1.5 or thicker or else the grease leaks out of the coupling. What test of performance (eg Timken OK, 4 ball wear, etc...) would best describe a grease's ability to prevent fretting of a splined shaft? What testing is used to quantify the shear resistance of a grease?

 

[Tmoose]

I think you'll get some replies that lubing splines is folly.

Please post pictures and drawings of the coupling construction, and the wear you (the spline really) are experiencing.

If the coupling or equipment manufacturer does not have anything useful to say I would have a two pronged approach.

First I'd get on the phone with the folks at Kluber armed with lots of detailed technical info. If their recommendation fits the budget I'd use it.

If the price was a little too breathtaking, when I regained consciousness, I'd get some FALK LTG.

https://www.rexnord.com/contentitems/techlibrary/documents/840-201_productsheet

Are the spline pieces removable for cleaning and inspecting?
If so I would be tempted to take used pieces still in excellent condition and shot peen them with fine glass bead, then gently smooth the wear surfaces with 400 grit wet or dry sand paper to create a plateaued finish, then scrub them with various clean solvents and a brass wire brush. Followed with a cleaning with solvent recommended by Dow Corning to prepare for applying their recommended Molykote spray dry lube product, either brush or spray.

 

[TugboatEng]

I tried the Kopflex KSG coupling grease but it was too thin and exited the coupling very quickly.

To describe the coupling, it is simply an SAE flange mount hydraulic pump. It has a DIN 35-40mm spline input shaft. It spins at up to 2300 rpm and transmits roughly 75-90 horsepower max (average 30ish). Opposite tbe splined end has a cavity with roughly the capacity for half a tube of grease and a plug There are no seals on the splined end. The coupling came from the mfg with what I'm guessing was Kopflex Waverly Torque or a similar moly based product. The datasheet specifically warns against use in high speed couplings. The issue I have is severe fretting wear. I have confirmed all of my dimensions met the requirements of the particular SAE flange mount. I did find the OE coupling to be quite soft, 37C, so I had some produced at 45-48C. The pump shaft is 55C. I am just now putting the harder couplings into service and want to make sure they're protected.

My ultimate goal is to find a grease that reliably provides 1 year of service (3000 hours) without significant wear of the pump shaft or coupling and without relubrication. Better would be to provide 30,000 hours of service because that's when we overhaul, anyways. Mobilgrease 28 has been a great improvement over any other products but I'd like just a bit more performance. I have found Huskey has a full line of polymer thickened greases (kind of like that Falk and Kopflex grease) in NGLI 2. I plan to contact them but am just trolling here for some insight first.

The Molykote pastry lubes are an interesting thought but it seems like the failure would be too rapid if the past lube fails. This component is critical to safe operation of our vessel.

 

[romke]

A polymer thickened grease is the way to go, to prevent fluid and thickener separation due to high centrifugal forces. That being said, it will not necessarily solve your fretting wear problem. Maybe a polymer thickened grease containing quite a bit solid lubricant like MoS2 or graphite in the basefluid might help. However, to get effective separation of surfaces that hardly move at all will be difficult to achieve with any form of more or less fluid or plastic lubricant.

 

[TugboatEng]

I feel MOS2 powder would also separate just like the soap thickeners.

The pump is a Rexroth A4VG hydraulic pump. The spline is fairly conventional for an oil lubricated PTO from an engine or transmission. My trouble is that mine is driven by belts so I have no available engine oil. There are no fretting issues with standard applications lubricated by regular engine oil so I don't think EP additives are necessary for long life. In fact, one manufacturer of a similar equipment used an oil lubricated system with a toothed belt to drive the same pump. The splines didn't wear as far as we know in the operation of those units but the rest of the execution was just awful.

 

[SuperSalad]

If it were me, I'd look for a polyurea based grease, for the shear protection/high speed application, with MoS₂ for the fretting. You need good boundary lubrication to prevent/slow down the fretting process in tight fits and both polyurea and MoS₂ should help there as long as you can get it delivered to the area where you're seeing the wear.

Andrew H.

http://www.mototribology.com