DT Topic 10
Terms in this set (51)
A good example of this is a person seen on the left. He is holding a stack of books on his back but he is not moving. The force downwards is standing still.
A good example of a dynamic load is the person on the right. He is carrying a weight of books but walking. The force is moving.
The person in the diagram is sat on the mono-bicycle and the air filled tyre is under great pressure. The air pressure inside it pushes back against his/her weight.
The factor by which a mechanism multiplies the force or torque applied to it.
The ratio of the distance moved by the point at which the effort is applied in a simple machine to the distance moved by the point at which the load is applied, in the same time.
All the calculations used for Mechanical Advantage (MA) and Velocity Ratio (VR) assume that the mechanism is 100% efficient:
Efficiency = MA/VR * 100%
Efficiency = 4/5 * 100% = 80%
The position and amount of input force applied to the lever.
The load(s) is/are interchangeable and by moving them to different points on the lever, different effects can be produced.
The fixed point of the lever about which it moves is known as the fulcrum
The output weight applied to the Lever.
Like a see-saw or balance, the load and the force are separated by the fulcrum. As one moves up the other moves down. The amount and the strength of the movement is proportional to the distance from the fulcrum
A wheel barrow is a second order lever. Here the load is between the force and the fulcrum. This uses mechanical advantage to ease lifting of a large weight.
Here the force is between the fulcrum and the load. Mechanical advantage is reduced but the movement at the load point is increased.
A structure at rest (or one that is moving at a constant velocity) is said to be in equilibrium.
Gears are used to change speed in rotational movement.
Type of motion measured in speed, and in one direction only. Together these make up the velocity.
Motion is back and forth. In the example to the left the reciprocating motion of the piston is converted to the rotary motion in the crank.
Motion in a circle. It is measured in either angular velocity, the number of degrees turned in a given time, or in revolutions per minute (rpm).
Back and forth motion about a pivot point and tends to be an ending point for a mechanism rather than the starting point.
Motion which starts and stops regularly and is usually the end result of a mechanism rather than the starting point for conversion.
Motion which has no obvious pattern to its movement. It is often needed in automata to recreate movement.
Gears that mesh together. Gear 'A' is called the 'driver' because this is turned by a motor. As gear 'A' turns it meshes with gear 'B' and it begins to turn as well. Gear 'B' is called the 'driven' gear.
DRIVER - IDLER - DRIVEN
A gear used so that the rotation of the two important gears is the same.
A gear used so that the final gear in a gear train rotates at the correct speed. In the diagram, gear 'A' is actually two gears attached to each other and they rotate around the same centre.
Rack & Pinion
The pinion rotates and moves the rack in a straight line - another way of describing this is to say 'rotary motion' changes to 'linear motion'.
A gear used to change the direction of drive in a gear system by 90 degrees. E.g. Hand drill. As the handle of the drill is turned in a vertical direction, the gears change the rotation of the chuck to a horizontal rotation.
A gear used to reduce speed. For each complete turn of the shaft the gear shaft advances only one tooth of the gear. Always drives the worm wheel round, it is never the opposite way round as the system tends to lock and jam.
This system uses pulleys to transmit motion and force from the driver shaft to the driven shaft. It fits tightly into the groove on the pulley wheels to keep slipping to a minimum.
A system that uses two gear wheels and a chain to transmit rotary motion over a distance. Compared to a traditional gear train, many gears must be arranged meshing with each other in order to transmit motion.
A systems used when there is a need to transmit rotary motion. E.g. A winch.
Distance moved by driver pulley / Distance moved by driven pulley.
-Motor DRIVER pulley diameter 200mm at 20RPM
-DRIVEN pulley diameter 400mm at 10RPM
= 400/200 = 2 or 1:2, DRIVEN:DRIVER
Where the pulley's wheel is stationary and does not move. E.g. A basic fixed pulley. It has a mechanical advantage of 1. Which means the Force input = the force output.
The Wheel or the pulley itself is movable. It allows free movement of the wheel / axel which let the movable pulley to have a mechanical advantage of 2. Meaning that: Force input = 2x force output.
Both movable and stationary/Fixed pulleys. By having several pulleys on the system, mechanical Advantage is furthermore improved.
An additional pulley which keeps the maximum amount of belt in contact with the driver and driven pulleys. Jockey wheels are often spring loaded and can also be found in chain and sprocket systems and used in a bicycle derailleur gear system to keep the chain tight.
A surface where there is a slope on the end points, or in other words on a surface where the height is different It is one of the six simple mechanisms. We all know that work done is force and distance, and by moving an object gradually on an inclined plane, less force is needed than lifting it up vertically.
When cutting hard materials, you can of course choose to crush it. However, to gain more precision and using less effort this can be used. Saws transfer the circular or linear motions of such inclined planes to the surface being cut and multiply the force applied.
A series of rigid links connected with joints to form a closed chain, or a series of closed chains. Each link has two or more joints, and the joints have various degrees of freedom to allow motion between the links. It is called a mechanism if two or more links are movable with respect to a fixed link.
A Linkage where the top rod moves to the left the bottom rod moves to the right. The bars move in opposite directions. Another way of describing this linkage is the direction of movement in one rod is reversed in the other rod. The fixed pivot is the centre of rotation.
As the large rod at the top of the diagram moves to the left the two small rods at the bottom move to the right. All the rods are parallel to each other.
Crank and Slide
The rods move forwards and backwards in slider. The fixed pivot anchor the linkages to one place.
This linkage allows horizontal movement to be converted to vertical movement. It also works the opposite way round. A practical example of this is the brake mechanism on a bicycle.
Moment or moment of force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.
Ratchet and Pawl
Has teeth cut out of it and a pawl that follows as the wheel turns. The ratchet wheel turns and the pawl falls into the 'dip' between the teeth. The ratchet wheel can only turn in one direction - in this case anticlockwise.
A fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure.
A component that changes the input motion, which is usually rotary motion (a rotating motion), to a reciprocating motion of the follower. They are found in many machines and toys
The shape of the cam- usually pear, circular, heart or drop. All are simple and smooth shapes and a sharp sudden profile would create a jam with the follower.
There are different types and they all slide or roll on the edge of the cam and move up or down depending on the Cam profile.
Acts like a lever, increasing mechanical advantage (the distance between the handle and the central shaft is increased - this makes it easier to turn).
Crank and Slider
The slider which slides inside the tube and the connecting rod which joins the parts together. As the slider moves to the right the connecting rod pushes the wheel round for the first 180 degrees of wheel rotation.
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