Many trextbooks will tell you to use the torsional capacity of an equivalent unkeyed shaft i.e. use the diameter measured from the circumference to the base of the keyway. It was supposed to cut out all the stress concentration factor mumbo jumbo. ■

Dear Mr. kzumbulev

Effect of keyway is accounted in calculation by application of weakening factor, in many of the methods.

You seem to be from USA; the American code for design of transmission shaft considers the weakening factor for keyway as 0.75. So, you have to reduce the modulus of section by this factor or some people do it other way around i.e. by keeping the modulus of section unchanged and applying this reduction factor on stress to be chosen.

Obviously the keyway application factor considers the shaft, hub, keyways and keys would be manufactured with dimension and tolerance as per accepted practices.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

My "Design Data For Shafts" from International Nickel suggests the following as "safe" :-

In torsion

Zt = PI * D * Dr^2 / 16

In bending

Z = PI * D * Dr^2 / 32

Where

D is the shaft diameter

Dr is the shaft depth under the key.

Of course you still have to ensure the key is sufficiently long to transmit the required torque! ■