Design of Power Screw

[marcuscouceiro]

Hello Gentlemen,

I was designated to create a tool for our maintenance depot and since I am new in the company I would like to show a good work by providing all necessary calculations and so on.

Basically we have a pinion with 65 mm OD (+0,024,+0,011 tolerances)that fits by interference in the 65 mm bore of SKF 7313 BECBM bearing. The top part of the shaft has a 5 mm 3/4 UNC threaded tap right in the center.

In order to install de shaft onto the bearing, the techs have to find a screw with the same thread mentioned before and it has also to be long because they install the screw in the shaft, and from the top of the gear box, they start to pull the shaft through the bore by turning a nut on the screw (sorry, do not know if made myself clear).

So, I would like to propose the creation of a power screw with one section made with 3/4 UNC and the rest with ACME threads.

Here we go:
- How do I calculate the necessary axial force to install the shaft onto the bearing?
- After calculating this force, how do I use it to chose the material of the power screw?

Thank you for your help,

Marcus

 

[MikeHalloran]

It sounds like you are reacting the pulling force through the bearing balls into the housing.

If so, THAT'S A REALLY BAD IDEA. Or perhaps I don't understand the assembly. A good drawing of what you are trying to do would be most beneficial.

You should find equations for the force in Machinery's Handbook.



Mike Halloran
Pembroke Pines, FL, USA

 

[marcuscouceiro]

Hi Mr. Mike,

Thank you for your answer. I have just uploaded a drawing to give you an idea.

What you said is correct. The inner ring of the bearing is really reacting to the pulling force but this is the correct way, according to the service manual.

Regarding the equations, is there any equation that relates the necessary axial pulling force to the interference? I mean, if the shaft was smaller in diameter the only thing I should take into consideration would be its weight but since I have the interference and that geranerates pressure in the material I could not find the correct approuch to solve this.

Thank you once again and sorry for English errors,

Marcus

 

[hydtools]

There are tools on the market that can do the work. For instance:

http://www.otctools.com/products/pullers_and_accessories

Reference the SKF bearing catalog or guide for assembly forces.

http://www.skf.com/portal/skf/home/products?newlink=first&lang=en

Ted