Stainless Steel track surface grade to match a 440C roller?

[Jeremy Sawatzky]

TL;DR Summary

I need to know if my 440C track roller will put an indentation in my SS304 surface when subjected to 2000lbs of force.

I have a machine I am designing that has a carriage supported by Track rollers on a machined flatbar track.

The load that each individual roller will experience could reach 2000lbs. worst case scenario.

The track surface is machined 304L - 1" wide as you can see in the green in the attached image.

The rollers are made from 440C and cover the full width of the 1" wide track. The roller diameter is 1.75" - the rollers are circled in red.

What calculation can I do that will show how much of an indentation that the track roller will push into the track surface? I am trying to plan if I need to attach a harder metal strip on top of the track to handle the load. - or if I am being overly cautious and I can reduce the width of the track for lower material costs.

Thank you!

 

[jrmichler]

If you search cam roller, you will find good information on track hardness. All of the cam roller manufacturers have engineering information that will answer your question. RBC Bearings is one such manufacturer, and their track hardness calculations are easy to follow: http://rbcbearings.com/camfollowers/engineeringdata.htm.

 

[Jeremy Sawatzky]

Thank you for the link, following the equation:

I come up with a minimum PSI rating of 122,000 psi - hardness of HRC 24

SS304L has a tensile strength of about 76,000 psi - hardness of HRC 1 - or HRB 82 - so its too soft for this application.

For the track then will probably look for a Duplex material - 2205 has a hardness of HRC 31 - 144,000 psiPlease point out if you see any flaws

Thanks!

 

[jrmichler]

I do not see any obvious flaws.

 

[Jeremy Sawatzky]

Thank you!

 

[Lnewqban]

Is that force of 2000 lbf per bearing or just the total load to be lifted?

I can see the difficulty of machining those four holes perfectly aligned as to have flat contact between each bearing and track and even distribution of the load on those four lines of contact.

 

[Baluncore]

I can see the difficulty of machining those four holes perfectly aligned as to have flat contact between each bearing and track and even distribution of the load on those four lines of contact.

It may be possible to employ plastic design, then it will self align once it is in use, and as it wears over time.

 

[jrmichler]

Roller alignment is part of the design of any track roller system. There are three main approaches:

1) Design for one roller to carry the entire load that should be carried by two rollers.

2) Machine all parts as accurately as possible, then design the structure so that elastic deflection will result in the loaded rollers having roughly equal loads.

3) Design roller adjustments into the structure. The rollers can be mounted in adjustable plates, the tracks can be adjustable, or adjustable rollers can be used. See adjustable cam roller in pages 18 and 19 in the RBC cam roller catalog: http://rbcbearings.com/literature/pdfs/RBC-CF.pdf. This is the preferred approach.

 

[berkeman]

See adjustable cam roller in pages 18 and 19 in the RBC cam roller catalog: http://rbcbearings.com/literature/pdfs/RBC-CF.pdf.

Interesting. What would you use the "crowned" OD rollers for? Are they crowned proud or dimpled in?

 

[Dr.D]

In this sort of context, crowned typically means both principal curvatures are convex.

 

[berkeman]

In this sort of context, crowned typically means both principal curvatures are convex.

Thanks. What applications would you choose crowned over cylindrical for?

 

[Dr.D]

Typically a roller follower (for a cam application) will deflect under load. The straight cylindrical surface will lead to heavy contact on one is and much less on the other. The crowned surface provides for more even load distribution and helps to avoid high stresses at the ends.

 

[Jeremy Sawatzky]

Thank you all for the input - yes this is an early model and does not yet include adjustment allowance for the cam rollers. I intend on making them adjustable to not only level the load but ensure equal loading.

I am also thinking that if I double up on the rollers, I would not need to add a harder track surface. However I would need to build a floating pivot to equally distribute the load on the 2 rollers. - the cost may outweigh the price of harder track material/machining.

Thank you!

 

[Baluncore]

In the OP picture, the rollers run on two faces of a rectangular rail that is mounted on a rigid frame. If those rails were elastic and could flex slightly, it would reduce the roller force adjustment requirement. The benefit would be gretest if the pitch of the rollers was different to the pitch of the rail mountings to the rigid frame.

The mountings of the rails on the rigid frame could be staggered, left to right so four points of contact did not match the four roller positions.

Avoiding equal mount and roller pitch, and the staggering of the left and right rail mountings would be a no cost design change. It would increase the resilience of the system, independent of the roller mounting adjustment.

 

[Jeremy Sawatzky]

To anyone referencing this thread. The machine is in operation. I ended up using v- groove track rollers. I ended up needing to use a material which hardness was about double that of SS304. I used lengths of 2205 duplex flatbar then had the long edges machined to a v-shape to match that of the roller. This worked great.