# Hex vs. Key

Our team is planning our drivetrain for next year, and we are considering using hex shafts instead of a key, as we have done in previous years. What are the pros and cons of each? I’ve heard that a hex broach costs quite a bit of money, but that’s about all I’ve heard. Also, how do hex shafts work with e-clips?

A couple more hex related questions:
-When we buy say 3/8" hex stock, is 3/8" the diameter of the incircle or the circumcircle?
-If we buy 3/8" hex stock, I’m assuming we would use a 3/8" hex broach? Would we drill a 3/8" hole, then broach it, or drill some smaller hole?
-We only have a manual lathe. If we wanted to turn down the ends of the hex shaft so that they fit into bearings, is the interrupted cut hard to deal with?

Thanks a lot!

Hex is measured flat to flat, not point to point. So, it is the same measurment as the inside circle. Before broaching a 3/8" hex, drill and ream a hole to 3/8". Most broaches are usually .0005-.001 oversize so 3/8" hex stock will be a slip fit into a 3/8" hex broached hole. A hex can easily be held in a 3 jaw chuck or in a hex collet. The “interrupted cut” is usually never a problem when turning it down on a lathe. Just don’t take too heavy of a cut at once.

When we buy say 3/8" hex stock, is 3/8" the diameter of the in circle or the circumcircle?

3/8 is the distance from flat to flat of the hex http://www.mcmaster.com/param/images/aluminum/hexbarsize.gif

-If we buy 3/8" hex stock, I’m assuming we would use a 3/8" hex broach?

yes.

Would we drill a 3/8" hole, then broach it, or drill some smaller hole?

depending on what broach you get there will be a specified pilot hole most will be the broach size rating, but check the broach you plan to buy. mcmaster part# 2875A19 does use a 3/8 pilot

We only have a manual lathe. If we wanted to turn down the ends of the hex shaft so that they fit into bearings, is the interrupted cut hard to deal with

interrupted cuts aren’t that bad, as long as you use a good cutter and the correct speed. The trick is just go slow, making small passes. This also depends on your material, if you are doing alum or steal.

There’s a lot of relevant discussion (including alternatives to buying a hex broach set) in this thread:
Chief Delphi Forums - Hex Broach Options Consensus is: buy the hex broaches, they’re easy to use and are very reliable.

Here’s a page from McMaster-Carr for broaches:
McMaster-Broaches
A 3/8" broach uses a 3/8" drill and creates a hex hole with the inscribed circle 0.376" across. A 3/8" shaft should fit snugly in this hole.

We have always used keys due to the simplicity and cost.

Most sprockets are keyed so why not use them.

Proud to say our TigerDrive transmissions are hexless and keyless… :ahh:

A manual lathe is perhaps preferable to a CNC one, since you can judge the cut and chatter, backing off as necessary. Take a light cut, and cut the shaft speed down about 30% from normal. It’s possible to break the tool, so have spares on hand, and take a lighter cut.

The key is an even feed rate sideways, so if you have a screw-cutting lathe, use the finest feed rate (usually about 0.001" available.

An alternative is to grind away some of the points first, just don’t overdo it.

Don

Without a doubt, if you can afford the hex broach, or find a shop willing to do it for you, use hex shafts wherever possible. It’s so much easier than using keys–nothing to lose, and unlike keys, the hex won’t slip.

The key will stay in place if it is brazed to the shaft, which immobilizes it (adhesives might work). You can also cut a light groove around the shaft for a retaining clip or e-clip.

Most gears and sprockets with finished bores have a standard keyway cut into the instead of square or hex shafts, when purchased from sources such as McMasterCarr and Grainger. One reason why keys are so common.

There is also the option of woodruff keys which are stronger and will not slide out, but I will admit, hex is nice.

The only thing that is kind of annoying about hex is when you go from round shaft down to a smaller hex. Then you have to mill it with an indexing head (or 4th axis in the case of CNC).

Nor will it weaken the shaft.

The keyway in a shaft is a stress concentrator, which significantly reduces the strength of the shaft, especially its resistance to breaking under shock loads. Keyways are popular 'cause they’re easy, and using a 5/8" shaft where a 3/8" will do isn’t an issue if weight is not a consideration.

Einstein says the limit is ~286000 miles per second
Dean says the limit is 120 lbs.

Don

isn’t the speed of light closer to 186,000 miles per second…

but regardless… I think key does pose alot more problems… i think hex shaft would significantly reduce maintainance and wear issues… we have lots of problems with keys walking out, shafts getting worn
if you have the tools i would think hex is the way to go

i wouldn’t know but i do know that key kinda sucks

Wikipedia sez: According to the currently prevailing definition, adopted in 1983, the speed of light <in vacuum> is exactly 299,792,458 metres per second. Since 1 mile = ~1.609344 kilometers, the speed of light is about 186,282.4 miles per second, or in more convenient units about 0.983571 feet per nanosecond. [digression] This gives an upper bound for the speed at which information can be transported using electromagnetism; e.g., lightguide. Actual transmission speed in fiber optic cable is about 30% slower than this, because the fiber has greater electrical permittivity than free space. [/digression]

Short FIRST-friendly summary: remember, Aluminum weighs about one-tenth of a pound per cubic inch, and light travels about one foot per nanosecond. The second approximation has almost no practical significance for FRC robot building. So back to the main topic now:

but regardless… I think key does pose alot more problems… i think hex shaft would significantly reduce maintainance and wear issues… we have lots of problems with keys walking out, shafts getting worn
if you have the tools i would think hex is the way to go

i wouldn’t know but i do know that key kinda sucks
We’ve been using keys for several years. I’ll be ordering a 3/8" hex broach soon. Summer is the time to practice new tricks.

Machanically speaking, a spline shaft is the best, but that is really hard to get and it is really hard to broach matching splines without laying out some money.