Design standards for shaft flats (D-shafts) 9

[aaansari97]

Hi everyone!

I'm designing a mechanism that makes use of shafts. I have two questions:

1.We are locking a 45 T, 0.5M gear in place on a dia 4 mm shaft using two flats (180 degrees apart) on the shaft and two setscrews. The torque is ~120mN.mm and we have already tested out this arrangement on a prototype.

My questions is if 2 flats 90 degrees apart are better than 2 flats 180 degrees apart? Our very experienced design consultant wants to go with the former (2 flats 90 degrees apart). Are there any standards which dictate how many flats, their depth and width, and their angular spacing there should be? The only informative literature I have found is from Misumi (

https://us.misumi-ec.com/pdf/fa/2010/p0103.pdf)

but this doesn't help us much.

Any leads will be appreciated.

2. My FEA and hand calculations tell me that the shafts are safe enough if made in 6061-T6 as compared to SS304 or SS316. In fact, 6061-T6 is safer for static load conditions. I know that steels are generally used for shafts but again are there other factors (such as fatigue considerations etc.) that I am missing?

Thanks!

 

[EdStainless]

If the pin is easy to replace then keep it softer.
Sorry for the confusion.
You should try to keep the materials about 5HRC different from each other.
The 200 SS are interesting, their initial hardness isn't very high but they work harden like crazy.
And remember that surface finish in the slot is very critical.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[electricpete]

LPS to Tmoose. Setscrews at 90 holds better than setscrews at 180.

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(2B)+(2B)' ?

 

[Tmoose]

Hi JBoggs,
"Those screws do not transmit torque, and lock the inner race to the shaft and serve their purely locational purpose well."

In applications where such a bearing is retained by setscrews at the drive end of belt driven equipment ( even wen the setscrews are at optimal angles) the radial and tangential belt forces try very hard each revolution to cause micromotions that cause the setscrews to burrow into the shaft and the screw points to wear. Loctite etc can help keep the setscrews from physically turning and loosening, but the micromotions at the setscrew tips and bore shaft interface will proceed unrelentingly.

The typical "slip" fit using commercial shafting by design results in a few .001" clearance between the race ID and shaft OD.
The result is the inner race tipping or cocking on the shaft, and even quicker set screw burrowing and some shaft>bearing fretting.
Even securely tightened setscrews, flats or drill points on the shaft can not prevent this process with pretty modest belt loads.
A few .001" greater shaft clearance significantly aggravates the effect, and hastens joint failure.
Genuine line-to-line or slight interference fits enhance the joint performance considerably but make the bearings difficult or impossible to assemble.
If the setscrews are not fully tightened the end comes even sooner.

Some companies have made a good business repairing and upgrading shafts whose set-screw retained bearings were incapable of handling the task assigned.

https://www.hitekbalancing.com/2010/11/04/air-handler-shaft-and-bearing-repair-and-noise-reduction/

The bearings using eccentric collars in place of setscrews are even less capable.