Why is my keyway width so small? what am I doing wrong here? 1

[Mech5656]

Hello All,

I am calculating the width of a keyway to use a key for a hub and a shaft. The key will fit half in the hub and half in the shaft to prevent the rotation of the hub inside the shaft.

Here are the values

Hub diameter=1.5"
which is about same as the ID of the shaft.
r=.75"
Length=5"
b=width=?

max Torque, which I am going to apply= 1800 in.lb
material SS316, yield = 42100 psi
shear stress= yield/FOS = 42100/1.5= 28066 psi


Now,
As we know
shear stress= Force/bL = T/bLr

b=T/shear stress*L*r= 1800/28066*5*.75= 0.0171 "

This looks too small compare to what I thought.
could you please help
thanks




Torque=

 

[MintJulep]

5 inches seem really long for a key in a 1.5 inch diameter shaft.

 

[Mech5656]

Thanks for your response.

I want to make sure the key goes over the whole surface of the shaft.
just for increase strength and symmetry.
So you are saying everything is good with these calculations, the width is very small because the length is big??

 

[MintJulep]

There you have it.

 

[Mech5656]

Thank you very much.

 

[MintJulep]

Key stock will be found in "standard" widths. Often presented with recommended key width vs. shaft diameter.

A more typical approach would be to pick a common size key for your shaft size and calculate the length that you need to transmit your torque.

 

[Jboggs]

I'm just curious why you feel the need to calculate the key size when it sounds like you have a fairly standard application. Why not just use the standard key size for the shaft? In fact, I think you might be asking for trouble of some type (slipping, galling, etc.) by using such a non-standard size. My gut tells me to stick with the standards in this case.

 

[desertfox]

Hi

There are standard key and key way sizes to select from see this link:-

http://www.roymech.co.uk/Useful_Tables/Keyways/keyways.htm

 

[Mech5656]

Jboggs:

don't I need calculations to see which standard size key to pick based on the dimensions and the material(SS) I have?

 

[Mechanical321]

desertfox:
Sorry i forgot to mention I am in US. We use inches not millimeters, Do you know something like this in inches.

 

[desertfox]

Try this link

http://www.hercus.com.au/uploads/29161/ufiles/Keysteel-Hercus_Cataloge_2008.pdf

This link is from the links on the bottom of the first link I posted.

 

[Mech5656]

deserfox:

Thanks for providing this chart. Now I have a question here. What I want to have 2 or 4 keys on the entire circumference of the hub. How would that effect these height and width. Do I divide the height and width by 2, If I want to have 2 on the opposite sides?

 

[desertfox]

Hi

No you keep the sizes as they are, why would you want four keys? The most I have seen is two.
If you use two keys its recommended that they are placed at 180 degrees apart.

It seems pointless having multiple keys because it's unlikely that the keys will all act to transfer the load simultaneously and in addition assembling four keys could be quite difficult.

If you can tell us exactly what it is your trying to do we might be able to help further but pleased give as much information in a single post as possible,otherwise we end up with extremely long threads and information is scattered over lots of posts when it could easily fit into one or two.

 

[tbuelna]

A 1.5" dia shaft/hub joint with a 5" long key is not a good idea. There is no way you will get anything close to even load distribution over a keyway with that L/D ratio. You might be able to get a fairly even load distribution using two diametrically opposed keyways, but using more than two keyways would not improve things.