Plug Thread Binding

[Vaporware]

We have a large diameter (11") bearing cap that is binding in assembly. It is threaded with 2" of Acme-3g with a pitch of .25 (11-4 ACME-3G) and I think we should go to a larger pitch. It is loaded, and holds the thrust bearing in place. Is this thread too fine for the diameter? The machinist's handbook shows a 5" diameter is commonly threaded with a .5 pitch. Is there anything we should consider when moving to larger threads? This is my first time dealing with acme threads and I wanted to ask to see if anyone has greater experience with this topic.

 

[MikeHalloran]

That's a very fine pitch for an Acme thread that big, but that doesn't explain the binding. You need to find out why it's binding before you consider changing the design.

The two most likely causes of binding are lead error and eccentricity. The manner in which the binding occurs might provide a clue. E.g. lead mismatch in perfectly round parts will cause the assembly torque to rise steadily as the thread is progressively engaged. Conversely, if the required torque peaks at certain angular positions and falls off at other positions, something is not as round as it needs to be.

There are only a bazillion other things that could be wrong, but you need to eliminate those two as a start.

It's time to buy some serious inspection tools and/or services.



Mike Halloran
Pembroke Pines, FL, USA

 

[Vaporware]

Thanks. I'll bring that up to my boss. I hadn't even considered that it might be casting/machining errors.

 

[eromlignod]

Can you just bolt it on around the flange?

Don
Kansas City

 

[Vaporware]

No because it is used to adjust the bearing end-float/loading and we don't want to have to machine the end cap to very tight tolerances to achieve that with a bolted design. There is a locking screw behind it.

 

[Screwman]

The other thing to check is the Pitch Diameter of the two parts. A lttle bit of error on the PD can put you into a zero or negative clearance situation pretty quickly.
You will need a set of pitch wires or balls to check this, but it is esential for quality control on threads.

 

[eromlignod]

This may be kind of side-stepping the question a little, but something well worth having is an old New Departure Handbook. They were a bearing company and their handbook includes a huge number of different ways to mount bearings and design bearing caps. I refer to it all the time. I believe they're out of business now, but you can still find copies of their handbook. Here's one on Ebay:

http://cgi.ebay.com/New-Departure-H...goryZ378QQssPageNameZWD1VQQrdZ1QQcmdZViewItem

It's worth the eight bucks.

Don
Kansas City

 

[CorBlimeyLimey]

Screwman ... are you saying women can't check threads?

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Helpful SW websites faq559-520​

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[mog69]

CorBlimeyLimey
Careful! Screwman said "or" not "and" !!! It makes a difference !