Simple machine elements  (Page 2/3)

As shown in , there are three different classes of levers defined by the relativepositions of the fulcrum, effort, and load. A first class lever has the fulcrum positioned between the effort and the load. Examples of first class levers include: abalance, a crow bar, and scissors. In a second class lever the load is placed between the fulcrum and the effort. Examples of second class levers include: awheelbarrow, a bottle opener, and a nutcracker. Third class levers place the effort between the fulcrum and the load. Examples of a third class lever are a hammer, afishing rod, and tweezers. Most machines that employ levers use a combination of several levers, often of differentclasses.

The wheel and axle

Both levers and the inclined plane lower the force required for a task at the price of having to apply that force over a longer distance. With wheels andaxles the same is true: a poweful force and movement of the axle is converted to a greater movement, but less force, at the circumference of the wheel. In a circular geometry, torque is a more useful concept than force and distance. You can learn more about torque here . The wheel and axle can be thought of as simply a circular lever, as shown in . Many common items rely on the wheel and axle such as the screwdriver, thesteering wheel, the wrench, and the faucet.

Gears and belts

A wheel and axle assembly becomes especially useful when combined with gears and belts. Gears can be used to change the direction or speed of movement, but changing the speed ofrotation inversely affects the force transmitted. A small gear meshed with a larger gear will turn faster, but withless force. There are four basic types of gears: spur gears, rack and pinion gears, bevel gears, and worm gears. Spur gears are probably the type of gear thatmost people picture when they hear the word. The two wheels are in the same plane (the axles are parallel). With rack and pinion gears there is one wheel and one rack, a flat toothed bar that converts the rotary motioninto linear motion. Bevel gears are also known as pinion and crown or pinion and ring gears. In bevelgears, two wheels intermesh at an angle changing the direction of rotation (the axles are not parallel); thespeed and force may also be modified, if desired. Worm gears involve one wheel gear (a pinion) and one shaft with a screw thread wrapped around it. Wormgears change the direction of motion as well as the speed and force. Belts work in the same manner as spur gears except that they do not change the direction ofmotion.

In both gears and belts, the speed and force is altered by the size of the two interactingwheels. In any pair, the bigger wheel always rotates more slowly, but with more force. On both the big and the small gear, the linearvelocity at the point of contact for the wheels is equal. If it was unequal and one gear were spinning faster than theother at the point of contact then it would rip the teeth right off of the other gear. As the circumference of thelarger gear is greater, a point on the outside of the larger gear must cover a greater distance than a point on thesmaller gear to complete a revolution. Therefore the smaller gear must complete more revolutions than the larger gear inthe same time span. (It's rotating more quickly.)