Design: Speed Controller

Method A:

Use of a brake acting on a disc on the shaft containing the sprocket. A disc is required to be clamped or machined onto the shaft to which an external force is applied in order to provide the counter torque. This force is most easily provided by a linear hydraulic actuator. Two options are available, use of actuators to control a flat disc brake pad which would move against the shaft disc and use of calipers as commonly seen on bicycles. These are shown in Figure 4.1 and Figure 4.2.
Both options could easily provide sufficient force to control the angular velocity of the shaft and therefore control the velocity of the load. In order to control the velocity of the falling bucket it is necessary to control the angular velocity of the shaft and therefore control the force of the brake. The calliper system has the advantage of being easier to set up and unless sufficient force cannot be created this will be used. It will be necessary to create a feedback system to control the velocity to at 0.7m/s. Three possible options of controlling the fluid pressure and braking force are considered.
Manually: Similarly to brakes working in a car. A simple hydraulic circuit can be set up. The operator can exert a force on one piston which would produce the force in the hydraulic fluid which produces a magnified force on the brake disc. The input force required by the operator to the primary piston can be reduced by use of a lever. This method has the disadvantage that the speed control is fully dependent on the competency of the operator who would be required to read a display of the speed (electronically gathered from the shaft speed) and make necessary adjustments to the brake force. It is however easy to set up and would require no power source. This can be seen in Figure 4.3
Use of a Hydraulic Pump: A hydraulic pump can be used to provide a pressure to the disc brakes. It would be necessary to use either an electric or petrol/diesel engine to provide power for the hydraulic pump. With an appropriate control system this would however provide accurate control of the brake. This has the major disadvantage of requiring a much more complex hydraulic circuit and use of an engine.
Use of small hydraulic actuator: An electrical actuator powered by a standard 12V battery can be used to control the hydraulic fluid pressures. A simple feedback control circuit can be developed to increase the braking force if the velocity is greater than 0.7m/s and reduced it if less. This would be usable for a whole range of loads. The Brake Pedal in Figure 4.3 would be operated by an electrically powered actuator. This would have very precise and accurate control of speed and with an easily portable power system. Care would have to be taken to ensure the battery did not run out whilst in use. If electrical mains power was available it could easily be used to power the actuator.

Method B: Use of Direct Counter Torque

If torque can be directly applied to the shaft the velocity can be controlled as seen in Figure 4.4. The difficulty is in applying this counter torque. A petrol engine in a typical setup cannot be used this way since the torque from the shaft would require the engine to run in the opposite direction it was producing torque and would cause the engine to stall. An engine braking method could be used but would be difficult to setup and would be highly inefficient. An electric motor could be used but would require electricity to be available all the time which is not possible. A gearbox would be required to couple the engine and the shaft which would add to the complexities of the design. If however it was setup correctly the load falling speed could be easily and safely controlled and flexible for other loads.

Method C: Use of Dampener

A disc can be manufactured into the shaft. A rod is attached to the edge of the disk and leads to an actuator. When the shaft moves around it creates a linear motion in the rod which forces the actuator inwards and then outwards. The energy of the shaft can be dissipated by the actuator and therefore slow it down. More than one dampener must be used otherwise the motion of the falling bucket would be highly uneven. Using this method it would be extremely difficult to control the velocity of the load and the motion of lighter loads. Lighter loads may not move at all if they cannot provide sufficient force to move the actuator. It would also be very difficult to stop the load once it is moving. It would have the advantage of not requiring an external power source as such.

Electrical Actuator Select

The electrical actuator needs to be able to provide up to 1400N and run from a 12V source. Hiwin Actuators provide a suitable actuator with drawings shown in Figure 6.1. The Lam 2A Actuator as seen on (http://www.hiwinactuators.co.uk/linear_actuators.html) can provide up to 1500N of thrust and can run on either a 24V or 12V battery.

Bearing Selection

Since the loads in the shaft are not high and the stresses are low the bearings forces do not need to be calculated and need only to be selected on size. The bearings fit on the shaft at locations, at a diameter of 45mm and a diameter of 50mm. Two deep ball groove bearings have been selected from the SKF catalogue and are shown in Figure 6.2 and Figure 6.3:

Seals

Actuator seals to be chosen from the Claron Catalogue (http://www.claron.co.uk). For the driving actuator with diameter 60mm Claron Actuator number CP236196 with an outer diameter of 60mm, inner diameter of 50mm and width of 8mm has been chosen. This seal has a maximum pressure rating of 150bar which is well above the required pressure in the system.
The coefficient of friction between the brake disc and brake pad depends upon the materials selected. Using the information provided at http://www.roymech.co.uk the materials selected are Kevlar for the brake pad and cast iron steel for the disc. Under dry conditions the coefficient of friction is approximately 0.3. This gives a maximum temperature of the brake pad of 300ºC and a maximum pressure of 3MPa which is well within the range required by this brake.
The brake must be capable of providing the force required to counteract the torque. For this it is first necessary to determine the distance between the centre of the disc and the centre of the brake pad.
Note that the circular nature of the steel disc causes the average distance of the brake pad from the centre to be lower than this value meaning a slightly higher force in reality will be used. This small uncertainty is taken into account when calculating the factor of safety.
This gives the force which is required to be produced by the brake pads. It is now necessary to calculate the force required by the primary actuator to produce this pressure.
This is the force which is required to lower a full load without accelerating. This method of calculation assumes that there is uniform pressure over the disc brake which as the pad wears may not be accurate. Using a uniform wear assumption may be a more appropriate. A factor of safety must be taken into account to combat these unknowns. The actuator sized must be capable of supplying enough force but approximately two times this value. Since the exact stresses on the actuator and machinery are unknown and the torque distance on the brake pad has been calculated imprecisely a factor safety of 2 should be used which means that the actuator should be able to provide a force of 672N. (http://www.roymech.co.uk)
It is also necessary that the brakes are used to stop the bucket. In case of slippage of the disc brakes leading to a sudden acceleration of the load the actuator must be able to supply an increased force.

For example:

If the brake slips for 1second the load will accelerate to 10m/s. In order to reduce the speed of this safely, for example in 1second then the acceleration required is 10m/s2. This approximately doubles the force required by the actuator. For this safety concerns it is advisable to increase the force the actuator can provide to 1344N.