Simple Machines: Gears, Velocity Ratios and Mechanical Advantage. - ppt download
state the relationship between mechanical advantage, velocity ratio and efficiency Physics Machines. Prove that efficiency of a machine is the ratio between actual mechanical advantage and velocity ratio. Or. State the relationship between mechanical advantage. The Efficiency (or inefficiency!) of a mechanical system can therefore be calculated by dividing its Mechanical Advantage (MA) by its Velocity.
Draw an obvious line in texts along the edge of the hypotenuse in each triangle. Roll each triangle cardboard around a pencil to make it like a screw. Compare the distance of the screw of each nail, analyse the difference between them. The principle of the screw nail is similar to the inclined plane in which a weight is pushed upward.
Although a screw is a spiral inclined plane, it acts like a second class lever. The screw's point is the fulcrum, where the thread meets a substance such as wood is the load, and the effort is applied to the head of the screw. Pitch of a screw, nut and bolt, screw jack 1. A screw is a simple type of machine, acting like an inclined plane rolled up in a helix. The pitch of the screw is the distance between the threads.
So for one revolution of the screw, it moves laterally through a screwed nut a distance equal to the pitch. The screw may be turned by a force applied tangentially at one end of an arm, with the other end of the arm is fixed to the screw.
The screw advances or withdraws through a screwed not or similar screwed appliance. Philips screws were designed to strip under high torque to protect the tool from damage. A screw jack is like an inclined plane wrapped around a cylinder.
A simple screw jack has a bench-mounted base and a turntable fitted with screw jack thread. A cord is wound around the periphery of the turntable. The free end of the cord is threaded over a pulley and then hangs vertically to support a load hanger. Weights added to the load hanger produce a torque on the system. Also, weights can be applied to the top of the turntable. To find the pitch, turn the platform around once and measure the change in vertical distance between the platform and the base.
Increase the effort until the platform just starts to turn. Make a simple lifting jack. Bore a hole through a block of wood to fit a carriage bolt. Select a bolt that is threaded nearly its entire length. Sink the head of the bolt in the wood, so that it is flush with the surface and nail a piece of board over it.
Over the projecting threads put a nut, then a washer and short piece of metal pipe. The inside diameter of the pipe must be slightly larger than the diameter of the bolt. By turning the nut with a wrench the device acts as a lifting jack. To make a model jack use two wooden blocks of about 10 cm length.
Drill a hole at the centre of the smaller block. Use a screw of more than twice of the block thickness. Make a groove with a knife at the centre of the other block so that the screw can be placed in the groove.
Use a screw with a six angle head or flat-head. Forcibly twist the screw through the hole on the smaller block.
Put the groove on the larger block on the screw cap. Put the two blocks together then tightly nail them together with small nails. Make sure that the screw does not rotate and it is not loose.
The contact surface between the two blocks is flat and level. If the screw is loose because the hole is too large, twist a nut to fix it. If it is very difficult to make the groove that can contains the screw cap, place cardboard or iron gaskets between the two blocks instead of the groove. The iron gaskets should be able to be just passed by the nails nailing the two blocks. Place the device upside down.
Twist a nut on the screw and put an iron gasket on the nut and cover a piece of sawed iron tube on the gasket. The inner diameter of the iron tube should be slightly larger than the screw and its outer diameter should be smaller than the gasket so that it can stay on the gasket, not to slip away.
The iron tube may is slightly shorter than the pole of the screw appearing. Rotate the nut and the tube will go up or down. When use the device to lift a heavy object, use a spanner to screw the nut and underlay the tube with a rigid board to reduce the pressure of the tube.
A real screw jack is upright like the model and when the screw is turned through one full turn the heavy is lifted up or down a distance equal to the pitch of its thread 2. Screw thread and pitch A screw may transfer between translation and rotation. The screw thread is a ridge in the form of a helix on a cylindrical core. The ridge may be triangular V-shape or square or round. If the screw thread is triangular, greater friction may be used.
Mechanical advantage - Wikipedia
If the screw thread is square, larger loads may be lifted. The distance between the adjacent threads of a screw or bolt is called pitch.
When you turn a screw through one full turn, it moves up or down a distance equal to the pitch of its thread. Different standard screw threads include the following: Screws turning in bolts are used to fasten things together.
Screw mechanisms are used to adjust the focus of camera lenses. A screw thread in a clamp movers the jaws of a the clamp together. Automobile screw jack In a car jack a screw passes through a nut carrying an arm that fits into the chassis of the automobile. Effort is applied to the lever above the screw. When the lever completes one turn, the load chassis rises a distance equal to the pitch of the screw, the distance between successive threads. Archimedes' screw is a hollow inclined screw so that rotation of the screw raises water.RELATIONSHIP BETWEEN EFFICIENCY MECHANICAL ADVANTAGE & VELOCITY RATIO
It was invented by Archimedes, but apparently was not invented in China. Propellers A propeller is used as a rotary propelling device in ships and aeroplanes. It has blades inclined planes radiating from a central hub to be inclined to the plane of rotation as they drive a helical path water or air.
The two parallel propellers of a ship turn outward when the ship moves "ahead'". The propellers in the fore and aft side of a modern passenger ship are called thrusters. They can be used to turn a ship in a circle, but are used to speed up docking and undocking without the need for assistance from tug boats. However, the authorities in some ports require tug assistance to be available for all ships entering or leaving the harbour.
Make a rotor from the lid of a drink-can. Roll the outer edge to avoid cuts. Draw the three blades on the lid. Make cuts first along the thick lines and then along the dotted lines. Remove the smaller sections leaving three blades.
Put the drink-can lid on a block of wood and cut out the shape with a chisel. Drill at the centre two 5 mm diameter holes 5 mm apart, then remove the little bridge of metal between them to make a central slot. To twist the wire, fold a 60 cm length in half with a large loop at the bend. Slip rod B through the loop and clamp the free ends close together in a vice. Then twist up the doubled piece to give a long uniform twist of angle about 20o to the axis. The holes in the rotor may need a little trimming so that it will spin freely up and down the twist.
Use a short tube made from tin plate that slides easily along the wire. Twist the blades so that the angle of the rotor blades gives lift when the rotor is spun by being pushed off the wire. The assembly has 3 parts: The wire, held vertically, 2. To launch this flying saucer, hold the arrangement steady above your head by the tube and strongly pull the wire twist down with the other hand.
Use different blade angles, or different numbers of blades to get the best flight effect. Hence more effort will be required to overcome a load. A machine that has no friction is called an ideal machine or a perfect machine. This is defined as the ratio of the distances moved by the effort and load in the same time interval. It is independent of friction. Efficiency Ef The efficiency Ef of a machine is defined as: This means that all the work done by the effort is wholly used to overcome the load.
In such practical machines, part of the effort applied is used to overcome frictional forces which are always present. Thus the useful work done by the machine is less than the work done by the effort on the machine.
Types of Machines The Lever The lever is one of the simplest machines known. With it we can overcome a large resistance by the application of a small force. It consists of a steel bar or rigid rod supported at the fulcrum or pivot about which it can rotate. An effort E applied at one point of the lever lifts a load L or overcomes a resistance at some other points.
The lever operates on the principle of moments. Types of Levers There are three classes of levers: First order, Second order and Third order. The classification is dependent on the relative positions of the effort, load and fulcrum. The First Class Lever In the first order of levers; the fulcrum or pivot is between the load and the effort.
Examples of such levers are i the crowbar, a pair of scissors or pincers, claw hammers and pliers. Velocity ration is usually greater than 1 but could be less or equal to 1.