What's all this fuss about transaxle bearing loads?

[doc724 924]

Help me to understand this.  From time to time, there are comments by RS members about which is better, cast iron wheel weights, loaded tires, suitcase weights, weight boxes etc.  Positions for against are sometimes supported by the comment to "not add any weight to the axle bearings"  Are these comments fact based or an urban legend?. 

 

The reason for bringing this up is that I recently read a post that advocated one position.  So I decided to check out the allowable loads for the typical Torrington needle bearings used on 1 1/8 inch transaxles.  (I made the assumption that this is the weakest bearing-could be a bad assumption).  The maximum allowable dynamic load for this bearing is 8130 pounds and the maximum allowable static load is 18,800 pounds.  Even a modest safety factor of 3 (recommended by Torrington) reduces these to 2710 and 6267 pounds respectively (per bearing).  Two bearings per tractor means that the load can be double. 

 

So then I checked the tensile strength of cast iron, assuming that the transaxle housing may be the weakest link.  The minimum tensile strength of cast iron is 20,000 pounds per square inch.  Now I did not go about calculating the allowable load on the transaxle, but the greatest load is a bending stress where the axle housings meet the transaxle body. However, the design is such that the allowable stress is way below the minimum tensile stress.

 

I have yet to see a WH that weighs anywhere close to the allowable loads that the needle bearings will support.  Perhaps if you cantilever a 300 pound weight box 2 feet from the axle, which results in 7200 inch pounds of bending moment that has to be counteracted by something (housing and bearings?).  Maybe that is enough to damage the bearings.

 

As a side note, the idea is adding fluid to the tires is probably OK even though rotational inertia is generally a bad thing the farther you are from the axle (results in sheared gear teeth).  Since the fluid is not solid (frozen), and the tractor speeds should be pretty slow, the liquid will slide along the tire/rim interface and have a minimal contribution to rotational inertia. 

 

So I guess I would like to hear from RS members regarding their actual experience with respect to bearings or transaxles being damaged by cast iron wheel weights, loaded tires, suitcase weights, weight boxes.

[Ed Kennell 35,748]

Thanks Don...a lot of good information there.     I can't help you with your question as I have never had an axle bearing fail and probably would not know what caused it if one did fail.

Although I am guessing most of these low speed bearing failures are due to improper  lubrication. 

It would be interesting to see bearing life expectancy data at different loads, with proper lubrication and no lubrication.

 

Edited January 10, 2016 by ekennell
rvsd

[KC9KAS 4,741]

@ekennell The "R" and the "D" are closer together on the key-board than the "V" and the "U"!

[SylvanLakeWH 24,120]

2 hours ago, doc724 said:

Help me to understand this.  From time to time, there are comments by RS members about which is better, cast iron wheel weights, loaded tires, suitcase weights, weight boxes etc.  Positions for against are sometimes supported by the comment to "not add any weight to the axle bearings"  Are these comments fact based or an urban legend?. 

 

The reason for bringing this up is that I recently read a post that advocated one position.  So I decided to check out the allowable loads for the typical Torrington needle bearings used on 1 1/8 inch transaxles.  (I made the assumption that this is the weakest bearing-could be a bad assumption).  The maximum allowable dynamic load for this bearing is 8130 pounds and the maximum allowable static load is 18,800 pounds.  Even a modest safety factor of 3 (recommended by Torrington) reduces these to 2710 and 6267 pounds respectively (per bearing).  Two bearings per tractor means that the load can be double. 

 

So then I checked the tensile strength of cast iron, assuming that the transaxle housing may be the weakest link.  The minimum tensile strength of cast iron is 20,000 pounds per square inch.  Now I did not go about calculating the allowable load on the transaxle, but the greatest load is a bending stress where the axle housings meet the transaxle body. However, the design is such that the allowable stress is way below the minimum tensile stress.

 

I have yet to see a WH that weighs anywhere close to the allowable loads that the needle bearings will support.  Perhaps if you cantilever a 300 pound weight box 2 feet from the axle, which results in 7200 inch pounds of bending moment that has to be counteracted by something (housing and bearings?).  Maybe that is enough to damage the bearings.

 

As a side note, the idea is adding fluid to the tires is probably OK even though rotational inertia is generally a bad thing the farther you are from the axle (results in sheared gear teeth).  Since the fluid is not solid (frozen), and the tractor speeds should be pretty slow, the liquid will slide along the tire/rim interface and have a minimal contribution to rotational inertia. 

 

So I guess I would like to hear from RS members regarding their actual experience with respect to bearings or transaxles being damaged by cast iron wheel weights, loaded tires, suitcase weights, weight boxes.

 

Thanks for asking this question. I too am interested, as I saw all the discussion yesterday on another thread. Some claimed if it isn't on the tires / wheels It might strain the bearings. I thought weight was weight. I have dumb bells on my hitch to add rear weight and it works great - vertical pancakes...

 

Looking forward to the answers to make sure I'm not a "dumb bell"...

 

[Ed Kennell 35,748]

3 hours ago, KC9KAS said:

@ekennell The "R" and the "D" are closer together on the key-board than the "V" and the "U"!

 OK...KC   You got me. 

 

 Sylvan, Any weight added to the tractor is transferred to the  ground through the axle bearings therefore increasing the radial load on the bearings.

  Question is, is the increased load significant  to the life of the bearing?       

Weight added to the wheel or tire does not add load to the axle bearings.

Edited January 10, 2016 by ekennell
axial to radial

[SylvanLakeWH 24,120]

12 minutes ago, ekennell said:

 OK...KC   You got me. 

 

 Sylvan, Any weight added to the tractor is transferred to the  ground through the axle bearings therefore increasing the axial load on the bearings.

  Question is, is the increased load significant  to the life of the bearing?       

Weight added to the wheel or tire does not add load to the axle bearings.

 

Thanks for the prompt and clear answer Ed. So given doc724's points, does it really matter if you add 50-100 pounds to a WH on the tractor and not the wheels? I'm on a c 105 with a front blade. Add my 200 lbs and I can't see where it's an issue...but again, those dumb bells...

[doc724 924]

Just a point of clarification, needle bearings can only transfer radial loads (loads perpendicular to the axis of the needles). Ball bearings can transfer axial and radial loads.  Tapered roller bearings like in the front wheels of your car also can handle axial and radial loads.

 

wrt Ed's question on life of needle bearings under proper lubrication, there is data in the design catalog.  I will post it tomorrow as well as max rpm for the needle bearings which are way above what a WH axle turns.

[Ed Kennell 35,748]

Right you are Don.  Weight on the tractor increases the radial not axial load on the axle bearing.     My error.

[JAinVA 4,619]

Doc 724 has introduced some valid points with figures to back the postion that carrying a weight rack on the bearings is not bad as presumed.I appreciate the information

and feel better about loading the transaxle with weights.My basic experience is with larger and much heavier machines.These tractors carry most of their weights at the wheels

by cast iron weights or loaded tires.I have seen pictures of  WH's with what looks to be half a ton of weight hanging off the back. I cringe at the sight.They might be able to handle the load but I think that I will try to keep the off the transaxle bearings.I have to fix them if they fail.JMHO,JAinVA   Jim

[WNYPCRepair 1,875]

I would be cautious in adding a bunch of weight hanging off the hitch though. I would worry about the hinged hitch (for lack of a better description) punching a hole through the transaxle on bumps. All the weight on the hitch will be concentrated on the small part of the hitch that rests against the transaxle, that can't be good

 

[SylvanLakeWH 24,120]

Much appreciate the added thoughts and observations.

 

Along with the dumb bells on my hitch, I may now need to pay attention to my wife and shed more than a few pounds around my own middle... Hey, at least I can blame radial load to the WH's transaxle...

[N3PUY 1,031]

Weights added to the hitch area will increase traction on a level surface BUT weights added to the wheels increase stability on a side hill.

[WNYPCRepair 1,875]

7 minutes ago, N3PUY said:

Weights added to the hitch area will increase traction on a level surface BUT weights added to the wheels increase stability on a side hill.

 

 

Good point

 

[Ed Kennell 35,748]

28 minutes ago, SylvanLakeWH said:

 

 

 and shed more than a few pounds around my own middle... Hey, at least I can blame radial load to the WH's transaxle...

Are we talkin radial bearings or radial spare tires here?  

[wallfish 15,918]

5 hours ago, doc724 said:

So I guess I would like to hear from RS members regarding their actual experience with respect to bearings or transaxles being damaged by cast iron wheel weights, loaded tires, suitcase weights, weight boxes.

 

One of my tractors has a constant and continuous hanging load of about 500lbs off the back and haven't had any issues with the trans so far after 6 or 7 years. (8 speed with 1 1/8 axles) Only problem I had was snapping an axle while using dual 10.5 wide tires on it.

[Sparky-(Admin) 19,537]

  I believe in loading the rear rims with weight for snow-plowing traction. I don't have fancy facts and figures to back up anything I say....its only my opinion... but nothing last forever and everything wears out at some point if its being used (even our beloved WH's). So in theory an axle bearing with less load and proper lubrication should outlast the same bearing with proper lubrication and more load.

   Wall fish has a back-hoe hanging off his machine so he has no choice as to how the weight is added to his machine but for snow duty/traction we have choices and I always opt for wheel/rim weight.

Mike......

[DennisThornton 4,769]

Just some thoughts based on reading and actual use.  I think it's important to know the differences in weighted tires/wheels and other loads and that weights address different issues with different effects on the center of gravity, loads around the axle and loads on the axle, better traction and counter-balancing.  Obviously if a WH can haul a backhoe around it can handle some serious on the axles load!  But that machine probably isn't getting much mowing time.  Same with the FEL machines.  Light weight tractors can mow OK but that's about it.  Got to have some weight to do serious work!

 

Toro recommends their weight box for certain reasons, not as an overall improvement for all tractors.  My 2 cents is to weight the tires and wheels for the best center of gravity and the first shot at getting more traction and then more weight where you've got to have it.  My NH with FEL is about worthless to me without something hanging off the back!  I've never had the FEL off but I'm guessing that if I had the backhoe on without the FEL I would not like my steering!

 

My 2 cents...

[ACman 7,506]

34 minutes ago, DennisThornton said:

Just some thoughts based on reading and actual use.  I think it's important to know the differences in weighted tires/wheels and other loads and that weights address different issues with different effects on the center of gravity, loads around the axle and loads on the axle, better traction and counter-balancing.  Obviously if a WH can haul a backhoe around it can handle some serious on the axles load!  But that machine probably isn't getting much mowing time.  Same with the FEL machines.  Light weight tractors can mow OK but that's about it.  Got to have some weight to do serious work!

 

Toro recommends their weight box for certain reasons, not as an overall improvement for all tractors.  My 2 cents is to weight the tires and wheels for the best center of gravity and the first shot at getting more traction and then more weight where you've got to have it.  My NH with FEL is about worthless to me without something hanging off the back!  I've never had the FEL off but I'm guessing that if I had the backhoe on without the FEL I would not like my steering!

 

My 2 cents...

I totally agree . My Kobuta BX2230 is the same way .

[doc724 924]

I am pleased to see that this topic has prompted a lot of discussion.  Anyway, going back to Ed's question of yesterday about bearing life...I am not a bearing designer and going through the Torrington design catalog was enough to make my head spin...and I am too old for that.  What is significant is that since these are drawn cup bearings, there is no inner race, the needles contact the shaft directly.  The outer race is a thin steel shell which is a light press fit into the transaxle.  In order for the bearing to survive at all, the load cannot be supported by one needle.  Proper lubrication is needed to support the needle and distribute the load to the other adjacent needles.  Not any different than in a engine where the journal bearings support the crankshaft, but the crank actually rides on the thin film of oil between the crank and the bearing.  Lose the engine oil and you get a spun bearing (assuming the rings do not seize first).

 

There is not enough heat developed in a WH transaxle to cause the transaxle to seize, however, if you lose lubrication, the contact forces will be high enough to cause the needles to spall (fracture under a compressive load).  The seals in these machines were never designed for a 30 year life (or more in some cases).  All that is needed is a small breech and the gear oil will start to seep out.  Not enough to leave a spot on the floor, but as it gets hot, the viscosity goes down and more will seep out.  Pretty soon your transaxle is dry and from there everything goes down hill.

 

I am in agreement with Ed, (im)proper lube, not excess weight is likely the cause of any bearing failures.  That being said, others have raised very good points about too much weight on the rear hitch.  The local stress is very high where the hitch meets the transaxle.  It is a point (a line at best) and the contact area is miniscule.  Stress is simply load divided by the area (and don't forget the multiplication factor for the lever arms on the rear hitch).  

 

thx to all who have joined this lively discussion 

 

 

[Martin 2,130]

12 hours ago, WNYPCRepair said:

I would be cautious in adding a bunch of weight hanging off the hitch though. I would worry about the hinged hitch (for lack of a better description) punching a hole through the transaxle on bumps. All the weight on the hitch will be concentrated on the small part of the hitch that rests against the transaxle, that can't be good

 

 

i think if anyone is going to add weight to a hitch then they need to forget about the standard hitch that you mention, they look like they are only designed for pulling a load, trailer etc. Down force through weight (and lets face it, you need to add a few hundred pounds, or whats the point?) added to the end of those hitches is eventually going to bend them beyond use. They also make the front end too light with steering etc....Ive moved my 18 ft trailer with one a few times and don't even like that because it doesn't transfer the weight back to the axle properly and makes the front end/steering too light.

Use a decent hitch like Scott's or similar, has the right weight transfer to the transaxle and beefy too....

 

 

[Martin 2,130]

10 hours ago, wallfish said:

 

One of my tractors has a constant and continuous hanging load of about 500lbs off the back and haven't had any issues with the trans so far after 6 or 7 years. (8 speed with 1 1/8 axles) Only problem I had was snapping an axle while using dual 10.5 wide tires on it.

 

John, you bring up a good point here, the weight on the bearings isn't really an issue with wear, but having more leverage further out along the axle line (due to weight and force of the extra wheel width) caused an axle breakage. Looks like you found a weakness with the axle when running duals. 

[AMC RULES 36,945]

Wonder John, were you traversing level ground when the axle broke? 

[doc724 924]

Wallfish

 

When you looked at the failed axle that happened when you were running duals-where did the failure occur?  Was it between the inner and outer bearing or outside of the outer (needle) bearing?  Was the fracture clean through (meaning the wheel fell off) or did you catch it before catastrophic failure?  Was the failure at a 90 degree angle to the axis of the axle or more like 45 degees?  Each of these pieces of information can determine if it was a failure due to bending or torsion.

[DennisThornton 4,769]

6 minutes ago, AMC RULES said:

Wonder John, were you traversing level ground when the axle broke? 

 

[WNYPCRepair 1,875]

32 minutes ago, Martin said:

 

i think if anyone is going to add weight to a hitch then they need to forget about the standard hitch that you mention, they look like they are only designed for pulling a load, trailer etc. Down force through weight (and lets face it, you need to add a few hundred pounds, or whats the point?) added to the end of those hitches is eventually going to bend them beyond use. They also make the front end too light with steering etc....Ive moved my 18 ft trailer with one a few times and don't even like that because it doesn't transfer the weight back to the axle properly and makes the front end/steering too light.

Use a decent hitch like Scott's or similar, has the right weight transfer to the transaxle and beefy too....

 

 

 

 

Completely agree. I've always thought that thin bar of steel from the hitch pressing (bouncing) into the transaxle was a bad design