This is a Dorris gear box, driven by a 25 HP, 1775 RPM motor, 3 shafts, overall reduction 25.4:1, and lip seals. OEM suggested lubrication – oil, ISO 150.

Due to seal leakage and hostile environment (very abrasive dust) someone had suggested 8 years ago switching to a SWEPCO 604 grease ( used for open gear applications, good adhesiveness, water resistant, has consistency of a motor grease) and have the gear box filled almost to the top.

Again, it was aimed to elimination of seal leakage and dust ingress. The gear boxes operated for 8 years at acceptable temperatures of 150F-160F on the box skin, improved seal performance and, therefore, this lubrication method was declared a success story and was being used for many years in other 20 gear boxes.

One of them failed lately. Failure mode: the box did not transmit torque. When the gear box was opened, gears were found wet, visibly lost their profile, walls had thick 0.5 inch layer of soap like residue, about 40% of the space was still filled with grease. Roller element bearings were obviously packed with grease originally designed to lubricate open gears and plowing through it at 1770RPM! But the most striking observation was looseness in the shafts. Pinion gear shaft had about 1/16" play! The bull gear also had about 1/32" of play. That is how much the abrasive mix of grease and dust apparently wore out the races in the bearings.

I wonder if somebody has ever seen such an innovation. After all, there was 8 years of service with this type of lubrication and no seal leaks.

Thanks.

This message has been edited. Last edited by: David_G,

Hm.. The oil was replaced with grease and almost filled to the top. This would be the extreme of overlubricating.
If they have packed the grease 8 years ago and never looked at it again, I would say that probably the other 20 gearboxes are on the way!!

Maybe just on this particular gearbox someone just dumped a truckload of grease in the gearbox

Normally they use only a brush

Hi,
Of late people have started using soft greases for the lubrication of gear boxes specially in conditions where leakages are high due to high temperatures. However, in such cases also there is called as life or drain intervel.I feel that in such cases, the condition of grease is to be checked at least every six months and depending upon the condion of the grease it needs to be decided whether it can be used further or not. In this case grease has thickened and resulted in abnormal wear.
Proper condition monitoring would avoid such failures.

Vijay

It's always better to not have the high temps ( but you knew that already).You don't have to worry about it with an engineered and enhanced oil that has superior friction control and excellent foam dispersion. I know the one but I promised myself I would try to be vendor neutral here ( I'm a vendor).Quite often our clients report up to 40* F less temp. in gearboxes and we got a great new one from a horizontal boring machine of 60* F drop in hydraulic oil.

Why did the seals leak?

D

quote:

Originally posted by RRS_Dave:
Why did the seals leak?

D

IMHO seals start to leak when things are overfilled (over lubrication), in a dusty environment this oil on the shaft mixes with dust etc.. and you have nice scouring paste eating the seal lips, eating the shaft, spitting the groove where seal lip meets the shaft,--->
resulting in more oil leakage, more topping up, more overfilling, , somebody got and filled the whole box with grease.
No more oil leaks, no more problems, no more complaints, case closed ...

quote:

No more oil leaks, no more problems, no more complaints, case closed ...

Seems to me like the box being trashed is definately a problem. I agree that overfilling is a big cause of leaking seals when mixed with an abrasive atmosphere. However, I know of gearboxes that have lasted a lot longer than 8 or 10 years that are heavily loaded and in a pretty bad atmosphere (aluminum plant). I'm not sure what they do in maintaining them, but I do know they had several different seal manufacturers looking at some of them. Next time I'm in there, if I can remember, I'll ask what seals they settled on. I think they had the Falk rep in too.

Dave

1. Why did the gearbox leak? Simply because it was worn out by a perfect lapping compound consisting of oil and silica.

2. To my knowledge ( I could be wrong,) using grease in gearboxes not a very common practice due to the fact that it is hard or even impossible to deliver lubricant to the gears and bearings by utilizing grease.

3. How would one replace grease in a gearbox if needed? Not very practical.

4. The problem here is not overfilling. After all it did work for some years without overheating. And who knows what is the right amount of grease needed? anyway?

Moral of the story: Treat the cause and not the symptom! Use proper seals!

Dave, I will appreciate seals info.

David

There are some grease products for non-enclosed gears, but to my knowledge the grease is applied with a brush, in the contrary to bury the gears alive in the grease and forget about them.
Opening a gearbox at least once a year to take a look inside (other word is inspection or PM) could have prevented this.

Some thoughts:

Except for the advantages in sealing, grease is in general not as effective a lubricant as oil. It is not as good at removing heat or removing oxidation products. The contact surfaces run hotter, oxidize the lubricant, and the oxidation products do not in general flow away from the contact area like they do in oil.

The fact that comparable housing temperatures are seen does not mean the contact surfaces are cooler. The grease may require a larger delta-T to transfer that heat and so for similar housing temperatures you have higher contact temperatrues. Particularly if you have an insulating layer of aged/oxidized grease built up on the inside of the housing.

In an oil-lubricatedhousing, you might sample periodically and/or change the oil. But for greased housings few people ever think about sampling or changing the grease.... even though the grease is expected to degrade much faster than would oil because it has more degradation mechanisms (for example hardening of thickener), perhaps higher temperature, and less removal of oxidation products from the contact area. It is in part because it is simply much more difficult or to change or sample the grease if it can be accomplished at all). But the absence of sampling/changing becomes yet another challenge for the long-term reliability grease lub'd machines compared to oil.