What is the efficiency of a Hobson's joint?

  • Thread starter Stormer
  • Start date
  • #1
66
11
A Hobson's joint is a kind of gearless angled drive. Most often a fixed 90 degree angle, but there is also versions with free moving joints in each axle so it can move to any angle up to a little over 90 degrees. It is today mostly just a novelty and there is very few applications actually using it because a bevel gear or similar is used in stead.

e51fd18fc2054d7d90084dcb73741735.jpg



But how efficient is actually this drive in practice? I know it depends on the friction of the bushings the axles ride in and the load, but what is a ballpark number? For example for some polished stainless axles running in PTFE or oilite bushings (there is usually to small of a space to fit ball bearing linear guides) with "moderate" loads and low speed? I have searched but not found any reliable numbers.
 

Answers and Replies

  • #2
2,332
673
I suggest that you work it out for yourself, initially based on frictionless connections. If you apply a torque T1 to the input, what is the output torque, T2? I have not worked this out, but I think you will find that it is angle dependent.
 
  • #3
66
11
initially based on frictionless connections
Will that not just make it 100% efficient? Just like all other gears assuming no friction...
 
  • #4
Baluncore
Science Advisor
9,152
3,691
The efficiency is not constant, but will be a square or an exponential function of the torque. That is because side forces on the links are compounded from one end of the sleeve to misalign the other.

There is a critical alignment tolerance at which it will lock rather than slide. That is the arctangent of the friction coefficient. If the angle is greater than the critical angle, the rod will stick and not slide in the sleeve. Since the rod force is always concentrated at the same two points, the surfaces of the sleeve and rod will wear rapidly, to approach the critical angle.

Poor lubrication will result in fatigue of the sliding link elbows. There is no redundancy.

I do not think this is a reliable power transfer device. There are more durable solutions that weigh less. It might be used to transfer orientation information, but it cannot be expected to do real work for any long period.
 
  • Like
Likes berkeman, Lnewqban and Spinnor

Related Threads on What is the efficiency of a Hobson's joint?

Replies
4
Views
864
  • Last Post
Replies
6
Views
1K
Replies
4
Views
673
Replies
2
Views
21K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
0
Views
2K
Top