Molybdenum Disulphide with bronze bearings

[Toby.R]

I’m running a Molybdenum Disulphide additive with the oil in the headstock of a Holbrook Model C lathe. This uses large bronze bearings adjusted to very small clearance and the manufacturers specified oil is an ISO 5 viscosity hydraulic oil which is very thin indeed.

The spindle runs direct drive from multiple V belts for higher speeds and has a back gear for lower speeds.

I presume that the dynamic oil film will work well at higher speeds but I was concerned that the thin oil film might break down at lower speeds and high load both in the bearings and on the teeth of the back gear (heavy large part – big cut).
I have added the Moly to give a degree of protection in those adverse conditions but now it’s been suggested to me that the Moly will attack copper bearing alloys.

I’ve looked online for real information on the subject. There are various forum contributions warning of dire consequences but little of any authority.

I discovered that sintered bronze bushes often incorporate Moly as a lubricant, that there are ready made bronze bushes with plugs of moly filled wax for lubrication and a reference on the Buntings Bearings site that their aluminium bronze can be lubricated with grease, graphite or molybdenum disulphide.
All that points to the Moly being quite safe.

Is there anyone here with knowledge on the subject (rather than hearsay) who can reassure me that the Moly in oil won’t attack the bronze. I’m pretty sure it’s OK but a little nervous now.

Cheers Toby

 

[Conrad Hoffman]

I can't give you a definitive answer, but I'm not aware of moly attacking anything. It's very inert. The usual caution concerns EP additives in gear oil attacking brass/bronze syncros on old manual transmissions. Thus GL-4 for old British sports cars instead of GL-5. Today most gear oils are rated for both, so the additives must be different. Back to the moly question, usually you don't use it with sintered bushings because it can clog the pores, or in systems where it can settle out and cause trouble, but other than that it seems fair game in terms of compatibility.

 

[Limy Sami]

All I can add is I've run moly in a wide variety of gearboxes over the years and not had any problems with bronze bushes etc

One application that comes to mind was a heavily (could say over) loaded worm & wheel box (steel worm, bronze wheel) 1425 rpm input ,.......... space was at a premium, so a larger box wasn't an option.

Anyway moly increased their service life by a factor of 3+

 

[davycrocket]

Back in 1965, I bought a new BMC austin 1100.
New fangled (For that era) Front wheel drive and combined engine gearbox oil.
After the first 500 miles service and oil change, I put in a tin of Molyslip.
The garage said "Thats your warranty gone if the synchro rings go bad in the (Manual) gearbox."
The car had 125,000 miles on it when I sold it and the gearbox was still quiet and had a still slick gearchange.

Then I had a 1971 new BMC1800, just like a giant sized Mini. Again transverse engine , front wheel drive.
The front wheel bearings went noisy regularly requiring replacement.
Yes you have guessed, finally washed the grease out of two new sets, filled with Moly grease, and never had to change one ever again.

Now (As a Horologist) I use Moly to lubricate the springs in clock barrels whenever I replace a spring.


Davycrocket

 

[Toby.R]

Thanks for the replies guys.
I run a Reliant Scimitar (for non Brits it's a car) and there was a club member, sadly no longer with us, who was so enthusiastic about the stuff that he got a large number of owners using the Moly absolutely everywhere. Used even in auto boxes and overdrives where the makers of the Moly say not to.
Wonderful results over large mileages and many years use in all applications, even with the wet clutches; nobody found any problem using it which is why I was surprised to hear there might be a problem with bronze as there are bronze synchro rings and gear selector forks in gearboxes. Indeed the Scimitar's achillies heel, the front suspension trunnions, are bronze and we all now use a soft moly grease in there with excellent results.

From what I read on the various vehicle forums, there seems to be confusion between Moly and chlorinated (?) EP additives which can be harmful to copper alloys. Moly being sometimes classed as an EP additive.

Limey's observation of getting 3+ times the life on highly loaded parts ties well with Alan Dean's (the late Moly enthusiast) experience when running heavy earth moving equipment in Australia. He was getting a service life of 2+ years from a Cat gearbox that only lasted 9 months without. I take it that sort of service life difference is only seen where the component is sufficiently highly stressed that the normal oil film breaks down.

Cheers Toby

 

[SAG 180]

I heard a description of a molybdenum sulphide - bronze bushing failure. This was in a very large rotating drum assembly used for roasting/calcining bauxite ore. The two bushes, one at at either end ran automatic oiler systems pumping oil into the bushes. One bushing at one end of the drum was run with the additive and one at the other end run without molybdenum disulphide.

This is a very big assembly with many tons of ore in it, it's kept rotating 24 hours a day and can't be started by the motors when fully loaded. The bushes carry a lot of weight and there is massive inertia to the assembly.

In short the molybdenum lubricated bush failed as the clearances were filled by a layer of burnished molybdenum sulphide that made the bushing run smoking hot. The plant had to make an unscheduled stoppage to replace the bush and they stopped using molybdenum disulphide after that.

I would guess the area loading of the bronze bush was the critical factor, allowing the steel journals to wipe the moly into the bronze but not get the extreme pressure lubrication of the moly disulphide. The line contact of steel roller bearings in a CAT gearbox would be higher and it seems to work there, loading seems to be the key factor.

 

[jdj]

I can't give you a definitive answer, but I'm not aware of moly attacking anything. It's very inert.

Thank you! Some people act like it's sulphuric acid! WRONG!

Jeff

 

[tdmidget]

SAG180, it sounds as though the problem in that case was particle size. To be effective Molybenum Disulphide should be colloidal size. If it packed into the oilite it waas undoubtedly a suspension, not a colloid. Think of it this way: a colloid is comparable to homogenized milk. A suspension is comparable to buttermilk with bits of butter floating in it. The butter ( Moly) plugs the pores where the homogenized milk would flow through.
My best experience with it was an occillating conveyor whose bearings were failing in a time frame of 1-6 mos. The moly waas good after a year and a half when I left the company.
The mechanism by which it works is that it chemically adheres to ferrous metals and can only be removed by physical means; ie: machining, abrasion, etc. It is especially good for sliding applications. It does not affect red metals and is usually found in the same ores as copper.

 

[SAG 180]

SAG180, it sounds as though the problem in that case was particle size. To be effective Molybenum Disulphide should be colloidal size. If it packed into the oilite it waas undoubtedly a suspension, not a colloid. Think of it this way: a colloid is comparable to homogenized milk. A suspension is comparable to buttermilk with bits of butter floating in it. The butter ( Moly) plugs the pores where the homogenized milk would flow through.
My best experience with it was an occillating conveyor whose bearings were failing in a time frame of 1-6 mos. The moly waas good after a year and a half when I left the company.
The mechanism by which it works is that it chemically adheres to ferrous metals and can only be removed by physical means; ie: machining, abrasion, etc. It is especially good for sliding applications. It does not affect red metals and is usually found in the same ores as copper.

I spoke to the guy today who used the moly disulphide, the ore calcination barrel has six pairs of trunnions with steel tyres along the barrel, each tyre has a 30 inch diameter hard bronze alloy bush possibly phosphor with nickel as they were nearly impossible to scrape.

The bushings are water cooled as the roasting bauxite ore raises the barrel to around 600 degrees C and the barrel carries a load of 800 metric tonnes of bauxite. Apparently the drum has to rotate for five days after emptying, during shutdown to allow it to cool off or it will permanently sag the heat softened steel.

In this guy's opinion a bronze bush can't achieve the necessary pressure to take advantage of molybdenum disulphide's properties without being damaged, iow bronze is just not hard enough. He also added if you want a bearing company engineer to hang up the phone on you, just tell them you are using moly disulphide with their steel bearings as there is such a narrow range of extreme applications where it is practical and recommended to use the stuff.

My understanding is the stuff only lubricates under very high loadings that can smear the disulphide across the bearing surfaces, otherwise it builds up on the surfaces like it did in the bronze bushings on the trunnions.

 

[Conrad Hoffman]

IMHO, if your lathe headstock is running at 600C you've got more problems than oil additives are going to solve!

 

[SAG 180]

IMHO, if your lathe headstock is running at 600C you've got more problems than oil additives are going to solve!

Did I say the water cooled bushes ran at 600 degrees C?, tell you what... you can run moly disulphide in whatever you like, go right ahead.
Without extreme pressure that a bronze bush can never achieve, moly disulphide does nothing at all, except in some certain cases build up and cause damage.

 

[Toby.R]

Reply from manufacturer

Thank you for your responses on this, I also contacted the leading UK manufacturer of Moly oil additives asking if Molyslip would attack bronze bearings and got the following reply.

"Thank you for your e-mail.
I regret that the short answer is NO.
The MoS2 will NOT affect the bronze bearings, but some of the additives in the finished product will.
Although the proportion of such additives is probably too small to have any major affect, we do not suggest you risk it."

As I know they are very conservative in their advice, probably to avoid any claims against them, I will continue to use Moly in my lathe’s headstock to take care of possible oil film breakdown when doing heavy slow jobs.

Cheers
Toby

 

[Conrad Hoffman]

No question that build up is bad, but if the extreme pressure requirement is true than almost all common applications of moly are useless or detrimental. I don't believe that, as it reduces friction and prevents wear in all manner of low pressure sliding surface applications too. One place I firmly believe it shouldn't be used is in precision rolling ball applications, like spindle bearings. I don't think any reputable spindle grease contains moly. One place where the stuff shines is CV joints, which usually have sliding/rolling balls, in fact I don't think they'd be practical without it. Lots of pressure there under some conditions!

CH

 

[Toby.R]

I should add that the product I use is a car gearbox oil additive.
Car gearboxes contain bronze selector forks, bronze synchro rings and often bronze bushes.

I know from my car club experience that the use of Molyslip doesn’t cause premature failure of these bronze components even over extended mileages, indeed rather the opposite is the case, plenty of anecdotal evidence that it increases gearbox life as well as making shifting smoother.

Cheers
Toby

 

[SAG 180]

Looks like I'll have to do more research on what causes buildup, the substance does work as a lubricant though it seems you really have to carefully select your application. It's not just MoS2 that does this, PTFE additives can also build up stringy deposits in bearings under the right conditions.

I have used a product in really worn Cleveland V8's called Molybond which was straight molybdenum disulphide in oil, it is supposed to build up on worn bores and parts. It really didn't seem to do much except quieten one clicking hydraulic lifter which was enough improvement for me.