How to calculate maximum torque capability on square drives ? ? ?

In summary, the end connection that will be used on the shaft must be able to withstand a torque of 80 Nm. The shaft itself is capable of withstanding more than this.
  • #1
DAVE1982
3
0
Hi all,

I'm hoping this is a relatively simple question, but the usual Google search has not really offered me any hints ?

I need to make a shaft with a connection at one end. The type of connection is not critical as the shaft it will connect to is to be specially made as well. So, I have no limiting factors of compatibility with existing components, only that of torque.

The shaft must be no larger that 16mm dia. which pretty much rules out 1/2" square drive, but I would like to be able to calculate max torque on a number of different sized squares or hex's. It would only be a guess, but I would imagine the calcs to include details about contact points and / or surface area ?

The maximum torque I expect to see is 80Nm

Any hints would be much appreciated.
 
Engineering news on Phys.org
  • #3
I did read that thread earlier today, but its not so much the shaft that I need to know about, more the end fitting.

I "think" I'm right in saying that the torque capabilities of a shaft are different to those of a square / hex. Again, I "think" I need a calculation which takes into account the number of contact points ?
 
  • #4
The amount of torque that a shaft can handle is related to the material properties of the shaft. The shear strength (or resistance) of the shaft is dependant on the ultimate tensile strength of the shaft. Normally this value is found empirically. You cannot apply more force to the end connection, what ever it is, than the shaft can handle. If you do, you will shear the shaft (or yield it).

Now that being said, the end connection may not be able to handle the amount of torque you are applying to it. Is that the question you are asking...how much torque can a square or hex shaped connection handle before stripping out?
 
  • #5
Yes, that's right. I believe the shaft is more than capable of withstanding the torque. The question is focused on the end connection and at what point it would be stripped out.

The problem lies in the fact that whatever connection method I choose (square drive / hex etc.) I must be able to machine the female form of the connection into a 16mm dia shaft, so I would like to be able to use a formula to estimate the torque capability of a number of connection types before I commit to costly CAD analysis. Unfortunately, the CAD analysis is a requirement of the job, but if I were able to have an "educated guess" at the best connection type I would be able to limit the amount of time that a FEA engineer sits at a screen costing me my precious pennies !
 
  • #6
The more contact area you have on the reaction points, the more torque will be tansmitted before slipping occurs.

This link will help you visualize this...

http://www.cs.cmu.edu/afs/cs/academic/class/16741-s05/www/Projects/Screwdrivers(SethKoterba).pdf
 
  • #7
Not sure where to get the info for this one besides calculating the moment for the hex manually. You may instead of looking for info on hex bores try to look for a spline bore formula. Only places I ever see hex bores used is agg applications so maybe you can try that in your search as well.
 
  • #8
You have to be careful with the assumptions you make with regards to non-circular cross sections in torsion. The elastic theory gets pretty nasty. However, there is our handy friend Roark's.

For a square drive of side dimension 2a, the angular deflection, [tex]\theta[/tex] and the max shear stress [tex]\tau[/tex] are the following:

[tex]\theta = \frac{TL}{KG}[/tex] where

[tex]T[/tex] = applied torque
[tex]L[/tex] = Length
[tex]K[/tex] = Calculated constant in this case [tex]k=2.25a^4[/tex]
[tex]G[/tex] = Modulus of rigidity

[tex]\tau_{max} = \frac{.601T}{a^3}[/tex]
Despite what many people think, the max shear stress is located at the mid point on each side! It is not located at the corners.
 
  • #9
Almost any square type end you make will have the force only on the corners . Thus, you have 1/2 the end-to-end distance as the moment arm. There might be a problem if the contact area is small and local pressure exceeds the plastic strength of the metal. If that is the case, you need some type of splined coupling or a welded one, or a tapered bushing, etc.
 
  • #10
I'm confused by your concern. 80 Nm is not a scary torque level unless you are using a plastic shaft or some other soft material. Torque wrenches with 3/8" square drive are routinely rated to 100 Nm so you should have no trouble with 16 mm.

Do select a high strength steel alloy. Square female holes are usually formed by broaching. Use coolant during machining it so you don't overheat the metal ("draw the temper"). You'll be fine.
 

1. What is maximum torque capability and why is it important in square drives?

Maximum torque capability refers to the maximum amount of torque that a square drive can handle without causing damage. It is important because exceeding the maximum torque capability can lead to failure of the square drive, causing costly repairs and potential safety hazards.

2. How do you calculate the maximum torque capability on square drives?

The maximum torque capability can be calculated by multiplying the maximum recommended torque value of the square drive by the safety factor. The safety factor is typically between 1.2 to 1.5, depending on the application and material of the square drive.

3. What factors affect the maximum torque capability of square drives?

The maximum torque capability of square drives can be affected by several factors, including the material and quality of the square drive, the type of fastener being used, the size and shape of the square drive, and the applied load and angle of rotation.

4. Can the maximum torque capability be improved on square drives?

Yes, the maximum torque capability can be improved on square drives by using higher quality materials, increasing the safety factor, and properly lubricating the square drive and fastener. Regular maintenance and proper usage can also help improve the maximum torque capability.

5. Are there any safety precautions to keep in mind when calculating the maximum torque capability on square drives?

Yes, it is important to always use the recommended torque value and safety factor when calculating the maximum torque capability. Additionally, make sure the square drive and fastener are in good condition and properly aligned before applying torque. Always wear appropriate personal protective equipment and follow proper safety procedures.

Similar threads

Replies
58
Views
5K
Replies
1
Views
4K
Replies
3
Views
802
  • Mechanical Engineering
Replies
5
Views
3K
  • Mechanical Engineering
Replies
2
Views
2K
Replies
3
Views
2K
Replies
2
Views
3K
Replies
7
Views
12K
  • Mechanical Engineering
Replies
2
Views
1K
Replies
13
Views
1K
Back
Top