View Single Post
  #19   Spotlight this post!  
Unread 16-01-2013, 18:52
Oldbikerider's Avatar
Oldbikerider Oldbikerider is offline
Registered User
AKA: Graham
FRC #3132 (Thunder Down Under)
Team Role: Mentor
 
Join Date: Jan 2012
Rookie Year: 2011
Location: Australia
Posts: 22
Oldbikerider will become famous soon enough
Re: tapping CIM axles?

Interesting Nate. There's nothing like some hard calculations to assess an engineering problem! Your numbers agree with calculations I've done in the past.

However the situation is more complex. With heat shrinking, not only do you need the half thou or so interference to achieve the required torque capacity, but you need some clearance to be able to get the hub over the shaft in the first place. So your 190F is not going to be sufficient. You'll probably need more than double that temperature, and then the heat soak problems into the motor start to be an issue. For a data point, my experience is that red heat is needed for a steel gear onto a room temperature shaft.

Ether's heat sink idea might help, but usually in this situation you're applying a bending load to the motor shaft in operation and you don't want excessive distance from the motor bearing to the hub. This is especially important when you're dealing with the relatively small motor shaft of 8mm on the CIM motor. So typically, there won't be room for a decent heat sink, or for that matter for a plate to facilitate removal of a poorly located hub by using a press.

Heating may also alter the heat treatment of the hub or gear, plus damage the surface finish.

Don't get me wrong, heat shrinking can be used, and is a standard production method, and works very well in tightly controlled production conditions. However, for school students, using often less than adequate workshop equipment, the Loctite 680 method is quicker, less messy, safer, less risky in terms of certainty of getting it right, probably stronger, doesn't risk over-stressing the hub, and probably cheaper.

But heating stuff up to red heat with a blow-torch is always a good way to have fun

Graham.
__________________
Graham
Mentor

FIRST® TEAM 3132
Thunder Down Under
Sydney, Australia