Bevel Gear
Let’s talk about bevel gears. A bevel gear is a gear cut on a cone shape instead of a flat wheel. It can carry rotation around a right angle—that is a 90-degree turn—and change the speed-power ratio at the same time. It can also reverse the direction a shaft turns. That is an important extra use.
Picture a set of three bevel gears. Two upper gears are the same size. They revolve on or with a power-rotated drive shaft. The lower gear is larger. It rotates more slowly on an axis at right angles to the horizontal shaft.
Follow the motion. The left-hand gear revolves in the same direction as the shaft. Its lower teeth move toward you. That makes the lower gear rotate from left to right. The lower gear then turns the right-hand gear so it turns oppositely from the shaft. Its lower teeth move away from you. You have made two right-angle turns in that gear set. The two upper gears, meshed with each other through the lower gear, turn in opposite directions at the same speed on the same shaft.
Now add two clutches to that horizontal shaft. Each clutch can lock one of the upper gears to the shaft separately. Engage the left-hand clutch, and the gears rotate as I just described. Disengage that clutch and engage the other, and the gears rotate the opposite way. So the lower gear, and any machinery driven by it, can rotate in either direction by engaging the proper clutch. It can also be left idle by disengaging both. You will see bevel sets like this in shovel swing and walk mechanisms.
Differentials
Next, look at a more complex set of bevel gears: the differential. You will find it in cars, trucks, and other machines. A differential is a gear assembly that lets two axles turn at different speeds while still receiving power. It consists of a ring gear driven by a pinion or a reducing gear set. The ring gear drives a pair of horizontal shafts—the axles—through a set of six bevel gears. Two of those bevels are splined to the inner ends of the axles. Splined means they are locked by ridges so they must turn with the axle. The other four are called spiders. They are mounted in opposed pairs in a case attached to the ring gear. The spiders rotate with the ring gear and turn the axle gears. The whole system revolves on the axis of the axles.
If the load on the two axles is the same, the spiders do not revolve individually. They only revolve with the case around the axles. If both axles were locked, none of these gears could turn. If one axle is locked, each opposed pair of spider gears will revolve in opposite directions around its own axis, in addition to revolving around the main axis. That own axis turns end over end with the ring gear. The spiders roll on the locked axle gear. The other axle continues to be driven by the rotation of the case. It is also turned by the turning spider gears, so its speed is doubled. If one axle is slowed, the other will speed up proportionately.
Differentials occasionally use spur gears instead of bevels. In a double reduction final drive, the ring gear turns a shaft carrying a small gear. That small gear meshes with a larger one on the differential carrier. This gives a greater gear reduction than driving the carrier directly from the ring. A two-speed differential has two different gear ratios, and the driver can select either one. The shift is usually vacuum or electric, with control by a push button from the gear shift ball.
A differential is essential for delivering power to a pair of wheels that may have to revolve at different speeds. Think of an automobile going around a curve. The outside wheels go farther than the inside ones. A differential is also used to steer machines and change gear ratios by proper application of brakes. But it has a disadvantage. It will deliver most of the power to the axle or unit that is disconnected or running free. Picture a car wheel on a piece of ice. It will spin while the wheel on pavement is not turned at all.
Non-Spin
There are a number of differentials that overcome this difficulty. They use automatically locking spiders, or worms and worm gears instead of bevel spiders. In those, some differential action still occurs on curves, but they automatically lock against spinning.
The spiders turn in oversize bushings. If one of the axles starts to spin freely because of a failure of traction, the difference in resistance between the two axles will force the spiders away from the hard-turning axle. That jams them against their bushings. They stop turning, and differential action ceases. If the resistance of the wheel with traction is enough so that more than four times as much power is transmitted to it as to the free-spinning axle, the spiders will turn in their bushings again. A dragging differential action resumes. Binding ceases as soon as both wheels have traction. This device should not interfere with the drive wheels revolving at different speeds on turns.
Planetary
Now move to a planetary or sun set of spur gears. It includes a central sun gear or pinion splined or keyed to a shaft or axle. There are two or more planet gears. There is also a stationary ring gear with internal teeth. The planet gears turn on shafts fastened to a carrier. That carrier is also called a spider or driving flange. For reduction in speed and increase in torque, power is supplied to the axle, and work is performed by members attached to or driven by the flange.
The planets are caused to rotate by the turning of the sun gear. That rotation forces them to walk around the toothed track formed by the ring gear. As they walk, they pull the carrier or spider around with them in the same direction the axle rotates. You find torque multiplication by dividing the number of teeth in the ring by the number of teeth in the sun gear and adding one. For example, a ring of 30 teeth and a sun pinion of 10 teeth would produce a torque multiplication of four. A planetary gear set with a fixed ring like this gives you a compact and sturdy means of multiplying shaft torque. If the ring and the spider can be either held or allowed to rotate, you can get brake and clutch action.
The old International cable scraper winch has a cable drum turned by a planet carrier. That carrier can be stopped by an external friction brake anchored to the outer case. The ring gear is bolted to the planetary housing. That housing is held or released by a similar brake. It is called a clutch because it is used as one. The sun gear is driven through reducing gears by the tractor’s power take-off shaft. It causes the planets to revolve on their pivots. If the ring gear is held by the clutch, the planets travel around inside it. That rotates the spider and the drum so as to wind in the attached cable. If the clutch band is released and the brake applied, the spider is held. The rotation of the planets then turns the ring gear. The ring gear is a non-working part and imposes no load. Such shifts can be made at full load without interruption of power.
When both clutch and brake bands are released, the spider, planets, and ring gear all turn freely. External line pull can then revolve the cable drum to unwind it. The control linkages are arranged so that the brake and clutch cannot be applied at the same time. A similar arrangement may be used to produce a two-speed transmission. If both the spider and the ring are connected through gears or chains to a single output shaft, you get a change of gear ratio or direction of rotation by directing power one way or the other with the two brakes.
Planet gears themselves are simple, compact, and rugged. They usually present few service problems until they or their bearings are nearly worn out. But adjustments of the numerous brakes and clutches used for shifting and controlling planetary systems are likely to be very tricky. You will need special instruction, or the guidance of a trained service man, the first few times you have to adjust or overhaul them.
Local Shop Note
A mechanic in Garrison, N.Y., told me about a job at a heavy equipment seminar in Albany, N.Y. It looked routine but turned out to be two problems, not one.
He had a crawler excavator that swung slowly and drifted when he stopped the swing, but only when the house was sitting on a side slope. On flat ground it seemed fine. That detail mattered. On a slope, gravity pulls on the house through the swing gearing, so a marginal part that flat ground hides gets exposed.
He pulled the swing motor and found the swing brake plates worn. The brake is spring-applied and hydraulically released, and it wasn’t holding cleanly. That explained the drift. But before he called it done, he checked the swing gearbox oil and found it glittery. He opened the planetary and found a worn sun gear and two planet gears with bad bearings.
The worn planetary was not what caused the drift. It was a second problem that would have failed later. He rebuilt the planetary with a new sun gear, planets, and bearings, replaced the brake, and set the swing relief to spec. The drift stopped and the oil stayed clean.
The lesson: when you find one bad part, keep looking. On any planetary set, check the sun, the planets, and the ring. Wear won’t change the ratio, which is still the ring teeth divided by the sun teeth, plus one. It shows up as noise, metal in the oil, and eventually a failure.