Shrink Fit calculations 2

[jeffcannon]

I am currently working a new project that will require a shrink fit to be used in two places. Unfortunately I do not have any experience in using these. We are pressing a steel (S45C) worm on to a steel shaft (dont have material picked). The worm OD is .650 nominal and the shaft OD is .3150 -.0002 -.0004 (8mm G5 tolerance). Knowing the torque this will be subjected to (approximately 27 in/lbs) how do I determine the interference necessary between the components. I have a basic understanding of how much I will have to heat the worm to get the ID to expand, but I really don't understand how to calculate the pressure created by the interference or how to convert that into a max torque value. I certainly don't expect anyone to do the math for me, but any pointers would be appreciated. As a side note I do have a copy of the machinery handbook which gave me the interference fits required for FN 1-4 fits, but there is no numerical explanation of the holding strength of these fits. Thanks in advance.

 

[unclesyd]

I think this is covered in "Mechanical Engineering Design" by J.E.Shigley.

 

[swertel]

Machinery's handbook has everything you need in it. I have the 25th edition and it is under the Allowances and Tolerances section, pages 637 - 641 (to start).

In the index, look up Fits. You can find the correct subtopics below that one.

--Scott

http://wertel.eng.pro

 

[israelkk]

Beware that even if you have the pressure you still have to assume friction coefficient. Do not be tempted to assume high friction coefficient otherwise, you will face very unpleasant cases of slip of the worm.

 

[flash3780]

Shigley's book will give you the technique for solving your problem. Also, you can calculate the amount of thermal expansion with the following formula: ? = R×?×(T - To)
Where,
? = change in radius
R = radius at room temperature
? = coefficient of thermal expansion
T = elevated temperature
To= room temperature

Good luck.

 

[desertfox]

Hi jeffcannon

Have a look at these sites:-

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm#Press_fits

http://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-calculator.htm

http://www.tribology-abc.com/calculators/e6_2.htm

desertfox

 

[eadwine]

Have you thought of using a shaft slot and key instead? This design can transmit much higher loads than an interference fit. There are assembly issues as well, with the shaft and key, all you need is an arbor press. With the interference fit ovens are needed. Also, when the worm wears out it is more easily replaced with the key design than the interference fit. There is also the issue of cracking the worm if the shaft and hole aren't machined exactly right. You will also find that environmental temps will affect the holding power and forces of transmission of interference fits.

 

[desertfox]

Hi jeffcannon

This pdf might also help

 

[GregLocock]

Using a key to transmit torques is generally a no-no. A key is designed to provide alignment. If you really want mechanical drive then a spline or taper is the way to do it.

Jeff -if you can get your shrink fit to work with a coefficient of friction of less than 0.2 then that is a great solution.





Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[eadwine]

Greg, For heavy loads I'd agree, but we're looking at only approx. 27 in/lbs. Also, splines and tapers are much more difficult too machine and get right and also cost much more to produce. Remember we're in the race to the bottom.

 

[jeffcannon]

First off, thanks for all the information I appreciate it greatly. As far as some of the other mfg methods, we currently use a spirol pin through the worm and shaft, which works well under lighter loads. The key approach as Greg mentioned does not work well for transmitting torque. Especially under life testing. The spline might work, unfortunately I am in a situation where the assembly is being done in China. The people pressing the worm on the shaft are not very experienced and there is a real possibility they will damage the PMDC motor if there is a heavy press involved. I also have a situation where our competitor uses this method, & the customer doesn't like change. So thats how I got here.

 

[israelkk]

If it is assembled to a motor shaft then press fit may hurt the bearings. Assuming the worm is long enough you can glue the worm to the shaft using a high strength glue.

One more option is a micro welding or the worm to the motor shaft.

 

[unclesyd]

You may have to print this paper out to use the formulas, but I think it may get you on track.

http://www.engr.colostate.edu/~dga/mech325/handouts/press_fit.pdf