Bronze Bushing Design

[Tyroneous]

Can anyone recommend a good reference source, where I can find information on bronze bushing design?

I have an application with (4) four 158" DIA sheave with a pressed-in bronze bushing, rotating on a steel shaft. One sheave is fixed and the remaining (3) three are floating. The problem is with the recommended operating clearances (max, min)? The sheaves were purchased about 10 years ago and there is very little information available and the supplier is not willing to give out any recommended running clearances or original (as purchased) clearances. The operating parameters are as follows:

RPM - 90 max
Sheave Weight - 5000lb
Applied Load - 14,000 lb
Shaft OD - 9in to 11in (Can't measure directly)
Bushing Material - SAE 65, Bronze (Best Guess)
Shaft Material - 4140 (Best Guess)
Lubrication - Synthetic Grease

Any help or additional insights are appreciated.

 

[MikeMech]

A general rule of thumb for bronze journal bushings is 0.001" clearance on the diameter per inch of shaft diameter up to the first 5" or so. On larger shafts, less. I'd make the bushing ID about 0.005" oversize (after press fit) and grease groove it appropriately. Use SAE 660 bronze.


Best Regards,
Mike
mike@usmech.net

 

[arto]

Shigley's "Mechanical Engineering Design" [McGraw-Hill 3rd ed 1977] Book has a lot on Journal Bearings that might help in chapter 10.

 

[motorworks]

Hi
Any "older" Boston Gear book as a great section on bushings
eddie

http://www.geocities.com/motorworksnf/123

 

[motorworks]

Hi
Any "older" Boston Gear book has a great section on bushings
eddie

http://www.geocities.com/motorworksnf/123

 

[Tyroneous]

Thanks for the sources of information, I will do a little digging and find the books noted above.

As far as my application noted above, I recently measured the clearances in the bushings amd found the three floating bushings to have a clearance of around .014" to .015". In my search for allowable recommended tolerances, I turned to a few machinists for a little insight into my problem. Their rule was 0.001" on the diameter per inch of shaft diameter but made no comment limiting this to specific size? As well, one vendor who had previously quoted on a similar application had a running clearance of .009" to .010" with a similar shaft, but I wanted to double check to make sure.

The next question I have yet to answer is, when are they considered worn out? There must be a recommended max clearance rule somewhere but the problem is finding it or are they worn out when they stop doing their intended job?

Thanks,
Tyroneous

 

[arto]

Maybe try ANSI B4.1 "Limits & Fits?" their descriptions might cover what you're trying to do/define.

They give .005/0.010 for "RC5" in this size range & 0.005/0.013 for "RC6" "Medium Running Fits" "higher running speeds or heavy jnl. pressures, or both"

RC7 "Free Running Fits" 0.008/0.016 'where accuracy is not essential" or "large temp. variations" or both

RC8 [0.012/0.025]& RC9[0.018/0.038] "Loose running Fits" "intended for use where wide commercial tolerances may be necessary, together with an allowance, on the external member"

 

In regards to determining the point at which the bearings ( not bushings, bushings do not have sliding points of contact) are worn, remove the sheave, thoroughly clean the bronze " journal bearing " and place a straight edge along it axially inside to determine if it is still flat, a worn bearing may have a belled out wear pattern. Just check for general consistent ID diameter measurements. If these are good, keep the clearance below 0.020 " total.

Does the sheave wander or act erratically??? probably last a lifetime at that speed...

 

[Tyroneous]

The ANSI "Limits & Fits" is definately a useful guideline for the application in question and it also backs up the information from other sources including machinists and the vendor who quoted on a similar application, Thanks.

Due to the time required to measure the bronze journal bearings with the machine disassembled, I had to measure the clearances with the sheaves on the shaft. I was unable to determine if the bearing was belled but I accounted for eccentricity by taking clearances at three different angular locations ( 0°, 144° & 288°). I measured the clearances to be relatively consistent, within .002" to .003" on all three bearings indicating a relatively consistent wear pattern. As well, the sheaves don't wander or act erratically but they do have a vibration issue which is the result of no static balancing during original manufacturing, but that's another issue. The sheave with the greatest imbalance did not show signs of increased wear so the vibrations doesn't seem to increase the wear of the bronze bearings.

Thanks,
Tyroneous

 

[MCM]

By taking several dimensions at several points on the existing bushings, the smallest diameter or the smallest clearance found must be the design clearance. This is true if no heavy vibrations detected.
The running clearances also depend on the operating enviroment. If foreign particles, e.g. sand, metal chips, are present then large clearance is preferable. If the bushings are meant for sealing purposes then you might think of using a smaller clearance. At the speed of 90 rpm I think your bushings probably will last for a very long time without wear even with some vibrations.