sketch or idea needed

[cole7011]

Help... lol kinda out of my depth when it comes to Mechanisms .... thus i need your help

i a sketch of the following

a gear wheel is to joined to a shaft so that it can move along the shaft but not rotate about it... how can this be done?

thanks

 

[EnglishMuffin]

There are literally dozens of ways to do this. Here are four, in order of increasing complexity, sophistication, and cost:

Shaft with a keyway, dog point set screw(s)in gear
Shaft with a key in a keyway (key attached with screws)
Splined shaft, splined hole in gear (many types of spline)
Polygon shaft, polygon hole in gear

You might also consider fixing the gear to the shaft and sliding the shaft in the support bearings instead.

 

[cole7011]

Shaft with a keyway, dog point set screw(s)in gear

can you give me a more detailed explanation of this

as well as Polygon shaft, polygon hole in gear method

thanks

 

[rmetzger]

The set screw in the gear prevents rotation by residing in the keyway but allows axial movement as it can slide along the length as long as its not locked down onto the shaft.

For the latter, think of a hexagon shaped bar (nut profile) with a gear with a hex hole through the center surrounding it. A bit of clearance will allow it to slide along the length of the bar but the sides on the hex will not allow free rotation. Similar to the idea of a box end wrench - it can easily slide onto a fastener axially but also can provide rotation and torque to the part when turned because of the interference between the shapes.

 

[strokersix]

If the axial sliding must occur simultaneously with torque transmission then the shape of the driving member is important. In other words, a square key or steep sided spline will slide much better than a hex which actually will lock in place and not slide at all. Think "pressure angle".

 

[EnglishMuffin]

cole7011:

Sorry I didn't respond immediately - been busy. Actually, although you can certainly do it with a hexagon shaped shaft as rmetzger says, I was actually thinking of the ultimate in cost and sophistication - the rarely seen "Polygon" profile - at one time a trade name - eg see

http://www.generalpolygon.com/index.htm

The classic one of these is a sort of triangular shape with rounded corners - probably approximating a "constant breadth" curve. Among other claimed advantages, it has a self-centering action when torque is applied. In the early days when these first came out, about 40 years ago, I recall a British machine tool manufacturer (I think it was Herbert) who built a large spindle gearbox with all the splined sliding-gear-carrying shafts replaced with these things. Although the promoters claim otherwise, it cost a fortune of course, with apparently very little benefit. You do find them on some engines I believe, on the end of the crankshaft.