Bearing wear on Stainless Steel Bar

[GTFiji]

I have an application where I am running a radial SS bearing (0.2" wide, 0.625" O.D.) over a SS bar flat in a linear motion. The bearing travels back and forth over a 2 ft. section. The bearing is loaded down onto the bar as it rolls with up to a maximum of 50 lbs of force. The bearing O.D. is SS 440C Rc 58-65. My 1st protoype bar was SS 303. After 10,000 cycles the bearing and shaft were loaded with fine metal chips. I am assuming the bearing is "picking up" the SS 303.

What Stainles Steel would hold up in this application?
What should the surface hardness target be for the SS bar?

The cycles are not continuous but it could see up to 200,000 cycles over several years of service life.

 

[MikeHalloran]

What you're doing amounts to performing a repeated hardness test on the 303, i.e. loading it in compression through what is theoretically zero contact area, again and again. Something has to give; the 303 yields enough to make the contact area finite, but then you move the indenter, and effectively push a wave of 303 ahead of the roller. You're working near the left end of an S-N curve.

I'm surprised that the radial bearing's outer race didn't crack. They're not really meant to be used as cam followers, which is pretty much what you seem to be doing. If you covered it with a nylon or brass tire, the 303 might survive. That's just a guess. If you want to be sure, analyze the contact stress for a variety of materials.






Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

GTFiji,

Can you provide some additional details on the application and environment? Why did you choose stainless steel for the bar and bearing? Stainless steel in contact with stainless steel is susceptible to galling, so you should probably investigate a different bar material. One possibility is a carbon steel bar (SAE 1045) that has been hardened (either quenched & tempered or induction hardened and tempered) with a surface hardness around 52 HRC (550 HV). This is a commonly used item for shafts subjected to wear, often plated with hard chromium. A typical chromium plating is 15-25 micrometers thick with a hardness > 900 HV. I am not sure if Type 440C stainless would have a similar galling problem against Cr plating. Can you use a lubricant at the interface?

 

[GTFiji]

TVP - The application is fitness related. The environment is indoor & non-corrosive (ie gyms. The bearing is an off the shelf bearing from boca bearing,

http://www.bocabearings.com/main1.aspx?p=product&id=2278&n=SR4-ZZEE

It comes in SS 440c, I choose it for the size. I run a 0.250" dowel through the middle and roll it across the bar. I actually have (8) bearings total running over one flat of the bar, (2) side by side per dowel pin. When I do the math I could get up to a 50 lb radial load per bearing but it will usually be less.

I choose stainless for the bar solely based on appearance (high quality, high tech appeal) and corrosion resistance. If a steel bar chrome plated could be cost effective and not degrade then it could be in play.

 

[GTFiji]

I forgot to mention lubrication would only be an option initially but not on a repeated basis. The rolling surface of the bar is exposed to users and could be grabbed. Why temper and harden the steel bar prior to adding the even harder chrome?

 

[DDRSS10]

The reason for hardening the bar before chrome that chrome is very intolerant of deformation, if the base material was to deform the chrome would flake off. In my opinion the bar should have a small flat area for the bearing to ride,or go with a slider shoe rather than a rolling element.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[GTFiji]

The bearing will roll on a flat surface. What about thin hard chrome such as an Armaloy product. I am told this type will not flake under the stress as a standard hard chrome might.