Retention of key under tapered bushing 1

[geesamand]

I have a simple question for a common situation that I can't find in any documentation.

We mount sprockets to a shaft, primarily using QD bushings. The key is an ANSI Class 1 fit or BS metric rectangular - both of which have a small width clearance on the shaft and hub. With QD there is a set screw to pin the key down and eliminate fretting and vibration.

With Taperlock there is nothing to keep the key from vibrating or walking. I ruled out Loc-tite as it must be disassembled in the field without heat - and I could stake each end of one keyslot and keep it trapped but that won't prevent fretting or vibration.

What do you do in the case of a clearance key without a locking set screw?

 

[MikeHalloran]

Loctite Pipe Sealant with Teflon works pretty well against fretting, yet can easily be disassembled cold.

Also, the lower strength grades of regular Loctite will set up hard but still not require heat for disassembly. They are typically colored blue.

In actuality, Loctite makes thousands of products, but at the retail/technician level, there's only blue/red/green in order of strength and permanence.

If you're doing this as an OEM, just call Henkel/Loctite for lots of expert free help.


Mike Halloran
Pembroke Pines, FL, USA

 

[dvd]

1. tap the end of the shaft and screw on a retainer disk that won't allow the key to walk out
2. round end keyway that doesn't come to end of shaft and retains the key

 

[Tmoose]

I checked a few installation pdfs online and was surprised only 1 mentioned the "drive" key in any detail at all, but still no recommendations for key retention.

http://www.gates.com/~/media/Files/...n/Manuals/Vol 53 No 8 TL QD Installation1.pdf

Have you called the manufacturer to see what the h*LL they are thinking?
I'd like to hear their story.

A well installed taperlock grips the shaft and coupling very securely. The power transmitting function of the key in service is completely secondary, so I would have no concerns about motion, micromotion and fretting. That is the complete opposite of any "normal" device with a slip fit retained by setscrews or eccentric collars, etc.

Even if after installation the key could not be slid axially with a few taps with a punch I would be looking at a redundant method of retaining the key. Wicking Loctite etc would likely work fine, but some mechanical retention would provide peace of mind. Some mechanics might use a small chisel or center punch to deform the end of the hub key seat or sides of the shaft keyseat and "stake" the key in place. Although Sometimes functional, that method never looks professional, so is hard to recommend.


1 - a key with a tall lug installed on the closed end on the keyway. Kind of like a gib head key, but without the taper, and installed on the shaft before the hub with the raised lug in the opposite direction.

http://www.me.metu.edu.tr/courses/me114/Lectures/loc_key_spr_riv_files/gib-head_key.gif

2 - A small screw installed in a tapped hole axially in the hub directly above the key seat, so the screw head blocks the key's axial motion.

http://www.blainefabrication.com/projects/2012_08_02/IMG_5284.JPG

3 - A "dutchman" screw installed in a tapped hole bridging the joint between the top of the key and the top of the keyseat.
Like the pins ( not screws) shown here.

http://www.roton.com/images/964_1.gif

 

[dvd]

I have asked the manufacturer (Dodge/Baldor) about this situation and they respond that the keys stay in place. I am sure that keys stay in place until they don't.

One more thought would be to cut the shaft long enough to allow a shaft collar to be used to retain the key - probably use a shaft collar with two set screws.

 

[geesamand]

Tmoose,

I agree with every single statement you provided. I was pretty convinced they'd mention the keys somewhere if I simply looked enough through documentation, but ultimately found little or nothing.

Using a hook/gib key or staking the key slots to trap the loose key strikes me as risky, since fretting or vibration can't be controlled.

dvd, thanks for contacting an mfr. I think my next step is to call TB Woods.

David

 

[geesamand]

TB Woods states that the bushing should close around the key when assembled and squeeze it on width to prevent movement. But with looser key fits this isn't assured.

When asked about what tolerance the key must be to ensure this occurs, I was advised to get key stock that is "oversized" (-0 / +.002" for example) rather than "undersized" (-.002 / +0), and dress the key to be a line-to-line fit. Looking in Machinery's Handbook, this corresponds to using an ANSI Class 2 key instead of an ANSI Class 1 key.

I also got the impression that end users who think about this kind of detail are few and far between.

Not sure how well I enjoy this answer, but I now have a stronger appreciation for the fact that a QD bushing has a set screw to prevent key movement, making key dressing unnecessary.

David