Shafts
Straight shafts do two important jobs: they support gears and pulleys, and they carry power from one place to another. Crankshafts are different—they take reciprocating, or back-and-forth, motion and turn it into rotation.
Most straight shafts you’ll see are solid. But some are hollow. A hollow shaft might be used to save weight, like in a truck drive shaft. Or it might be hollow so that another shaft can turn independently inside it. Shafts can have the same diameter all the way along, or they can be stepped or tapered at bearing or attachment points.
The problems you’ll run into with shafts include twisting and breakage, misalignment, whipping and vibration, wear at friction and contact points, end play, and damage to keyways and splines.
Tight Fits
Here’s something you need to know about metal: all metals, even the finest, hardest alloy steels, will change shape slightly under heavy pressure. So if a gear or hub is fitted snugly on a shaft, both parts can distort under rotating load. When that happens, the contact surfaces work against each other, gradually wearing away the metal and causing increasing looseness.
To prevent this working of a gear hub and a straight shaft against each other, you can make the hub slightly undersize. That way, heavy pressure is required to force it onto the shaft. The undersize is calculated so that the compression of the metal equals the strain that will be placed on it. That way, destructive working won’t occur even under the heaviest loads. The rule is simple: the greater the strain, the tighter the fit.
You can also get a shaft that’s step ground with two or more different diameters. This serves two purposes. First, it avoids the need to press a gear or bearing inner ring a long distance along the shaft. The part goes on loosely until it reaches the step that fits it. Second, it provides exact location and solid backing for parts pressed onto those steps.
Now, a tapered hub’s tightness on a matching tapered shaft depends on installation pressure. The end of the shaft may be threaded, so you force the hub on by tightening a large nut. There’s no problem of sliding it along the shaft, and during disassembly you can make it finger loose by moving it off a tiny fraction of an inch. But that loosening may require great force. And remember: tapers must match to hold properly.
When a shaft and hub need to turn together, you have to prevent them from turning separately. A round hub gets a very poor grip on a round shaft, no matter how tightly they’re pressed together. The three methods most often used are keys, splines, and flanges. For small parts with light loads, you can use set screws.
Keys
A key is a square or half-moon shaped piece of hard steel. A keyway is a square-sided slot. You cut one in the hub and one in the shaft. The key goes in the shaft keyway (usually), and the matching slot in the hub slides over it. The key should be a finger-tight fit in the keyways, and the hub should be as tight as necessary on the shaft. Any looseness lets the hub work back and forth against the key. Eventually, if the load is heavy enough, that destroys both the key and the keyway.
The Woodruff, or half-moon, key is easier to use than the square key. And never use ordinary mild steel or any soft metal for a key. It will bend and shear off unless the loads are light.
Splines
A splined shaft has a number of grooves, similar to keyways, cut at evenly spaced intervals around it. Matching slots are cut in the hub, and the hub is slid or pressed onto the splines on the shaft.
If the splines are cut with clearance to make a sliding fit, you can control the position of the hub on the splined area with a clutch-type collar and yoke. If they’re a tight or press fit, the hub may be held in place by friction, thrust collars, snap rings, set screws, and various other means.
Flange
One end of a shaft can be greatly expanded into a round, flat plate called a flange. It’s drilled with a number of holes near the rim so you can bolt it to a hub or another flange.
Automobile and truck drive axles have splines at the inner, or differential, end and a flange at the outer, or hub, end. The spline may be a sliding fit, which lets you pull out the axle without opening the differential.
Set Screw
Set screws are used in light service. You drill a hole through the hub and a short distance into the shaft. The hub section is threaded, and you turn a bolt or screw into it until the end is firmly seated in the socket in the shaft.
Most of your set screws will be the Allen type. They’re short cylinders of hard metal, threaded from end to end. The bottom is tapered slightly, and the top has a hexagonal, or six-sided, socket. You turn it with an L-shaped Allen wrench. Where the strain is greater than normal or the screw is hard to reach, the threaded hole may be deep enough so you can use two screws, with the upper one locking the lower one. Whenever you remove an Allen set screw, check automatically to see if there’s another one below it. Otherwise you may waste a lot of time and damage a part.
Local Shop Note:
I remember a conversation with a heavy equipment tech in Rhinebeck, N.Y., who showed me why you never assume the first thing you find is the actual problem. I met him at a regional equipment seminar in Albany, N.Y., and he told me about an excavator that came in with a complaint of weak travel on one track and a knocking noise under load. He checked the final drive first and found the splines on the drive motor output shaft were worn. Replaced the motor, ran it through its functions in the yard, and the weak travel was still there on that side. So he went back in and pulled the track drive sprocket and found the splines on the final drive input shaft were also worn thin. That was the real culprit. The motor splines were worn, sure, but they weren’t the source of the weak travel. He replaced the final drive, and the travel and noise were gone. The takeaway from that job was simple: don’t stop at the first worn part you find. On a shaft, the damage you can see might be a symptom, not the cause. Follow the power path all the way through before you call it fixed.
Friction & Efficiency
There’s friction in every device that has moving parts. How much depends on design, material, construction, lubrication, and load. We’ll discuss the factors affecting friction in a later section.
The efficiency—more exactly, the mechanical efficiency—of a device is the percentage of the power put into a machine that can be taken out of it to do work. Output power is input power minus friction.
In a well-built, well-lubricated chain hoist, you might lose 3½ percent of your muscle power in each pulley. If there are three pulleys, the efficiency would be 90 percent. And you’d have to pull with 33½ pounds of force to lift 600 pounds with a 20-to-1 ratio, or mechanical advantage.
With inferior construction, poor lubrication, and heavy load, friction could easily consume 60 percent of your energy, for an efficiency of 40 percent. You’d then have to pull 75 pounds to lift the load. And if friction took 95 percent of the power, the advantage of the hoist would be lost entirely.
Even though friction is very important, we won’t mention it often in our discussion of leverage, so we don’t confuse the basic principles. Just keep in mind that part of your power is lost to friction, until we get to a detailed discussion of just how and where it’s lost.