Spherical Plain Bearings or Stay with Spherical Rollers

I would help to have the brand, model number and any detail drawings you might have...what leads you to believe that plain bearings are more robust?

http://www.treelinetampa.com/raycobearings.html

Hi SolarEagle,

Thanks for your response, are you requesting details of the bearings in use or of the stump grinder.

It is major surgery to remove, adjust, replace, and realign the cutter wheel each time bearings need re-adjustment which is why I am exploring the use of spherical plain bearings in the hope that owing to their simplicity and modern materials they may have a longer service life than spherical plummer blocks.

Plain (friction) bearings are typically far more robust that similar sized roller (antifriction) bearings.

The main reason is that contact surface area is far greater for similar sized plain bearings as opposed to roller bearings. The tradeoff, of course, is greater friction.

Yes, there are spherical roller pillow block bearings. Timken makes a line of spherical roller bearing plummer blocks (see page 95 of their catalog). These bearings will be more expensive to purchase than a spherical plain pillow block, but are very rugged and are made with "high-capacity bearings to meet the toughest demands of industry". They also have several different sealing options, including labyrinth, but the Taconite seals may be a better option for your application.

It is a little difficult without seeing the design of your stump grinder to know if these are even an option for you or not.

Hi 8320,

I am aware of Timken bearings but prefer to use Rexnord because they have a threaded screw-in ring adjustment to take up roller/race wear. It is major surgery to remove, adjust, replace, and realign the cutter wheel each time bearings need adjustment which is why I am exploring the use of spherical plain bearings. Are Taconite seals of a lip contact type with a spring over the lip? If so they will not hold up to the grit attack as conventional lip type seals do not. This is why labyrinth seals are currently in use.

Plain bearings are a much older technology, and could be more robust in your application. Getting a plain bearing design that can handle the thrust associated with the grinding, and also handle the radial load might be difficult, but worth exploring.

The speed range of the cutter wheel to ensure the correct lubricant film strength using grease could be tricky.

You may want to take a few temperature shots while in operation, if there is too much grease in the bearing, it will run hot and wear out prematurely. With a varying load will come a varying temperature, and finding the right grease to establish the right film could be difficult, it may be that oil lubrication could actually improve your operation.

RE.: Plain bearings are a much older technology, and could be more robust in your application. Getting a plain bearing design that can handle the thrust associated with the grinding, and also handle the radial load might be difficult, but worth exploring.

The speed range of the cutter wheel to ensure the correct lubricant film strength using grease could be tricky.

You may want to take a few temperature shots while in operation, if there is too much grease in the bearing, it will run hot and wear out prematurely. With a varying load will come a varying temperature, and finding the right grease to establish the right film could be difficult, it may be that oil lubrication could actually improve your operation.

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Hi RWilliams,

Thanks for your response. The act of grinding through a tree stump is relatively a steady load with variation between cuts at each sweep of the cutter wheel across the side of the stump. Increased load is experienced if the cutter wheel is in soil grinding away at the root system. I took temperature shots with the bearings running at 95-115C. I currently use a grease mixture of lithium EP2 complex, moly, and have recently added a high government spec. calcium grease to the mixture.

Hi again RWilliams,

Further, cutter wheel rotates at 1400 rpm. Labyrinth seals are in use for three reasons:

1. They allow grease leakage without back pressure thus not adding to frictional temperature.

2. they are more robust than lip seals

3. Owing to ingress of soil grit they allow the bearings to be purged of contaminated grease again, without creating a back pressure.

A pressurised oil delivery system sounds expensive on a mobile machine.

I was thinking an oil bath. You probably have enough surface area for your duty cycle to keep the temperature down. Inpro makes sealing systems that are used in heavy dirty industry, many OEMs do custom machining to handle their products.

https://www.inpro-seal.com/en_US/

Temperatures over 100C sound high, especially for the outside. Maybe the fit/preload of the bearing has to be adjusted for your operating load...

Do you meant gravity drip feed from a central reservoir at some high point. Plumbing will need to be resistant to impact.

Thanks for the heads up with inpro-seal. I am running Rexnord (LinkBelt) bearings and unless inpro has already designed a seal that I can purchase from a retailer they would not be interested in li'l ol' me, as their focus will be on high volumes presented to them by OEM's.

Oil in the bearing housing, self contained, they give you a little sight glass to know where to keep the oil volume, at some standard attitude of the housing.

You might get better heat transfer with the liquid, especially with splashing. The bearing manufacturer has probably already done the testing to know the heat dissipation values, and tell you what the delta in operating margins might be, oil to grease.

Further, I actually measured 95C on a cool day and probably overestimated an approximation of 115c for a hot summers day. I do not pre-load the bearings. I simply remove as much play as possible with very slight play remaining.

It could be that the lack of preload and the play left in is contributing to higher temps and shortened life of roller bearings.

Without preload, even with little to no appreciable free play when not in operation, the lack of rigidity in operation can result in increased vibration. Additionally any freeplay upon impact can provide room for momentum gains leading to far more severe bearing duty and shorter bearing life.

You may find that new bearings installed with slight preload run at lower temps in typical operating conditions. Of course, too much preload would heat things up as well.

Sorry for the delay--been away. Pre-load is very subjective when applied by feel. I guess its "feeling" how much there is in a new bearing and replicating it.

"... I currently use a grease mixture of lithium EP2 complex, moly, and have recently added a high government spec. calcium grease to the mixture...."

.

In addition to play left in/lack of preload, untested grease mixes may be contributing to higher temps/shorter life of bearings.

It isn't enough to just check one compatability chart. Different charts often contradict one another. The desire to 'throw a litte of this' should be resisted unless specific testing of that specific mixture of the actual greases has been completed.

Would it be worth looking at the bearing/seal combinations used by other stump grinder manufacturers, and speaking with people who use them to get an idea of other options already employed in the field and their durability?

Further thought - particularly hire operators, since they will be very interested in minimising maintenance (ie cost) and maximising availability,life and durability(ie income)

Good points and food for thought. Thanks for that.