A pulley is a lifting mechanism that makes it easier to lift a heavy load. It is made of a wheel with a groove between the two flanges around a wheel’s circumference. A rope, toothed , a smooth belt or a chain usually runs in the groove.

Pulley and belt drives are used in many household , such as sewing machines and vacuum cleaners. The different types of pulley systems include:  a single wheel fixed pulley,  a single wheel moveable pulley and  a compound pulley.

A single wheel fixed pulley has a fixed , meaning the axle is anchored or fixed in place. It is used to change the direction of the force on a rope (or a belt). Has no MA because the force is equal on both sides of the pulley and there is no force multiplication. A single wheel moveable pulley has a free axle. This means that the axle can move in space.

A moveable pulley changes the direction of the force on a rope (or belt) and is used to multiply forces. Two ropes are pulling on the object.

If one end of the rope is anchored, pulling on the other end of the rope will apply a doubled force to the object attached to the pulley. MA is greater than 1 (MA > 1). A combination of a fixed and moveable pulley system. This is an easier way to lift a heavy load. The top pulley is fixed and cannot move. The other pulley is held by the rope and hooked onto the weight (object) and it can move. The block and tackle is a compound pulley where several pulleys are mounted on the same axle in a compact design. This saves space and further increases the MA.

Block and tackles usually lift objects with a mechanical advantage greater than two. Mechanical systems and control Control is the way in which systems are regulated. The process of a system is influenced by control components (parts) in order to get the required output.

Ratchet and pawl Gears are generally used to change movement and this also includes being used to prevent movement or allow movement in one direction only. If the ratchet wheel has 8 teeth, then for every eight turns of the crank, the ratchet wheel will turn one complete revolution. An example is a ratchet screwdriver. The pawl is a special mechanism that provides additional safety to lifting systems.

It fits tightly into the teeth of the ratchet.

The shape of the ratchet allows the pawl to slip over the teeth when it moves in one direction.

A hydraulic front A disc brake is made up of a metal disc attached to the wheel hub that rotates with the wheel. The callipers are attached to the frame or fork and have pads that squeeze together on the disc. As the pads rub against the disc, the wheel and the bicycle slow down.

 The braking surface is at the top of the wheel which is away from the ground and prevents a build-up of debris and water on the rim of the disc and pads

 Disc brakes are reliable under heavy braking compared to rim brakes and the heat is transferred onto the tyre

 Wheel rims tend to be made of lightweight metal. Brake discs and pads are harder and can accept higher maximum loads.

 Disc brakes do not create wear on the rim, unlike rim brakes, especially if grit becomes embedded in the brake pads.  Disc brakes allow a bicycle with a buckled wheel to be ridden, which is not possible with a rim brake as the buckled wheel binds on the brake pads.

 A disc brake is easier and cheaper to replace than a wheel rim or drum.  A cast iron disc attached to the wheel

 Brake pads which apply resistance on both sides of the wheel

 A hydraulic piston which pushes each pad against the disc 1. Mechanical brakes Use cable disc brakes. The pads exert pressure on the disc rather than on the wheel rim. They require regular service but are not expensive to maintain. 2. Hydraulic brakes This system works on the principle of hydraulic fluid. Pressure is applied to the pistons which in turn applies equal force to the brake pads. A cleat is the naval term for a device with two projections pointing in opposite directions to which a rope can be tied to secure a boat next to the jetty. Cleats are generally horn shaped. They are controlling devices in pulley systems. They act as safety devices by controlling the movement of a rope. 1. What are the parts of a pulley system? 2. Describe a single wheel fixed pulley system and state what MA is obtained. 3. Why does a single wheel moveable pulley require a free-moving axle? 4. Does the compound pulley provide force multiplication? 5. What are the advantages of disc brakes? 6. What are the uses of cleats?