Set Screw Configuration 1

[DDIC]

I read somewhere (either Marks, Machinery's, or somewhere else) that one set screw on a shaft is stronger than two 180 degrees apart and that two 90 degrees (or 60 I can't recall) is stronger than just one.

Does anyone know any reference material where I can find this stated? I've looked all over Marks and Machinery's and scoured the internet but I can't find anything. I know I've read some material on this topic before.

 

[DDIC]

After some more research I found one article with claims but it gives no references so to me its just of opinion.

http://machinedesign.com/article/setscrews-1115

 

[drawoh]

DDIC,

I have a Stock Drive Products data book with torque figures for individual set screws. There is no mention of how to use multiple set screws. There is a warning that set screws marr your shaft.

I have had a lot of bad experiences with set screws. If it is important enough that you have to do calculations, you should select some other locking device. I like the locking collars out of the W.M.Berg, Nordex, PIC and Stock Drive catalogues.

JHG

 

[DDIC]

Well I work in the oilfield and the general use is to lock down a threaded connection where a roll pin cannot be used. So I would like the strongest connection (still being able to disassemble for repair) possible due to possible jarring and torque.

 

[JohnRBaker]

I agree with the issue of marring your shafts.

Back in a previous life (designing capital machinery for the baking industry) we pretty well did away with all use of set screws in any sort of power-transmission application. Instead we used what was called 'Taper-Lock' bushings (I've still got an old Rexnord catalog from 1980 laying around). Our first attempt at replacing set screws in power-transmission applications was something called a "QD" Bushing, but they proved to be a problem both because they took up more room and they were a notoriously poor choice where sanitation was an issue (remember we produced commercial bakery equipment, such as dough mixers, proofers, ovens, coolers, conveyorized pan and product handling systems, etc.), but with the advent of the 'Taper-Lock' busing we moved exclusively to that as it was both compact and much better in term of meeting sanitation requirements (but still worse than traditional hubs, but we all have to make compromises sometimes). Also, the use of these hubs made disassemble much easier for maintenance and repair, which is critical when you consider the amount of money these machines cost and they were expected to last a 'life time' (one of my first jobs when I was co-oping while in college was to work repair-orders, often on machines which were 40 or 50 years old).

An example of a typical 'Taper-Lock' bushing:

While this is what a "QD" bushing installation looks like:

While you can see why we considered the "QD" less sanitary, they did have an advantage of having more gripping 'power' and they could be installed with the hub facing either IN or OUT, while the 'Taper-Lock' had be installed with the bushing on the OUT-facing side of your sprocket/pulley.

Here's a short video showing how to install and remove a 'Taper-Lock' bushing:

http://www.youtube.com/watch?v=RBjtuetw44Q

DISCLAIMER: I have NO interest in any of the manufactures of these patented and trademarked products.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[Tmoose]

2 at 180 is looking for trouble. The hub can be riding up on 2 points, which makes it vulnerable to micromotions that progress quickly to fretting and looseness, especially if the load changes direction, like a belt drive sheave/pulley.

To be secure the shaft has to be pushed hard over to one side of the bore, so there is substantial rigid support, similar to being in a form fitted V-block.
Some companies that make pillow block bearings used to claim that their choice of angle between two set screws was "better" than the other guys'. For my money securing a bearing or hub to a shaft with any setscrew or eccentric collar is just looking for trouble in the presence of varying side load (belt drive, etc). It will wiggle loose after a few million load reversals and make a mess. Keyed hubs offer no better resistance to micromotion. A few thousandths extra clearance as commonly results with commercial shaft makes it MUCH worse. It is no coincidence that API and others insist on interference fits on coupling hubs much bigger than my finger. Tough to assemble, but that is what it takes to reliably transmit power.

HoloKrome and others talk about a setscrew's ability to resist axial load. Usually 1000 lbs plus or minus a lot.

 

[MikeHalloran]

I will only allow setscrews for use as Dutch Pins.
(Sort of like the Taper-lok without the taper.)


Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

Tmoose is correct. The drive is coming from the contact between the shaft and the bore on the opposite side away from the set-screw. The set screw just provides the force to push it against the opposite side. Two set-screws at 60 degrees or 90 degrees are better than one only because you have a back-up if one comes loose. It might also allow you to get the same force with smaller screws if you have limited space. We use set-screws in many applications. But, as the many responses suggest, there are a lot of other methods that are better for many applications. I did not have a reference to “prove” that two are better than one.

Johnny Pellin

 

[MintJulep]

Set screws work well enough.

To hold door knobs on.

 

[Tmoose]

The handle on our combination storm door mounts on a square shaft . It is secured to the shaft with a brass set screw ( 7 or 8 mm?) with a straight slot for a screwdriver tightening. The handle loosened a few times a year. For a while As a courtesy to the manufacturer I would tighten the setscrew as firmly as I dared. On a cold sleety evening this past fall the handle pulled off again, so the next morning I went to Ace hardware and got a (Chinese) black oxide finish steel socket head serrated cup point set screw, and installed it with a little moly grease on the threads to protect against rust and also provide greater clamping force for torque applied.

If I had worried that the set screw's bite damage to the shaft would interfere with removing the handle I would have drilled a shallow hole on the surface of the shaft where the set screw bites. Some applications drill a deeper shallow hole in the shaft and use a dog point set screw. That may provide better positive mechanical retention of the hub, but would not prevent the micromotion that will fret and wear the shaft hub interface.

 

[Cockroach]

You can also drop balls into a circular recess and lock the Box to the Pin in a circular groove, then plug the opening with a pipe plug. I've used this on Rotary Connections were torsion vibration is an issue.

Regards,
Cockroach

 

[KENAT]

Surely using 2 screws appropriately positioned also approximates 3 points of contact better to limit movement which will in turn reduce loosening - or am I missing something?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

KENAT,

A cup point set screw digs into the material, taking it up above its yield point. Any further force, like trying to transmit torque, takes it further above yield. Set screws mar the material, and they come loose.

There are all sorts of mechanisms out there, of all sizes, that wrap around the shaft and squeeze it. They provide much more clamping force, and far lower stresses. They cost more, but your system will not fail out in the field.

JHG

 

[KENAT]

Wasn't arguing that there aren't better coupling methods.

Just thinking through part of why 2 set screws at a few degrees from each other may be better than one.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[JohnRBaker]

How about:

It creates THREE points of contact, an inherently more stable arrangenment, perhaps even allowing you to place less pressure on the points of the TWO screws which might help reduce the potential for marring the surface of the shaft.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[DAVIDSTECKER]

I don't what machines you are worknig with but if you have access to the end of the shaft you might look into tapered, Gib Head keys.
They are driven in like a wedge and can be removed with a crow bar.