31-07-2012, 11:06 AM
MOTORIZED SCREW JACK
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INTRODUCTION
Fabrication of a motorized screw jack is easy especially when the parts are available in the market. This mechanical engineering project can be easily completed by integrating an electric motor with a screw jack.
A screw jack or a Jackscrew is operated by turning a lead screw. The height of the jack is adjusted by turning the lead screw. This can be done either manually or by integrating an electric motor with it. This integration is our project.
The difficult part in the project may be finding a low speed motor that is able to work at 12V. This is because the battery output of an automobile is 12V, and the electricity needed for the operation of the screw jack is taken from this battery. Another problem will be regarding speed reduction. 12V motors usually operate at higher speeds, likely at 4000 or 5300 rpm. So reducing this high rpm to the required lower rpms for the operation of screw jack without bulky accessories or power loss can be challenging. But still this is one of the projects in mechanical Engineering.
THEORETICAL CONSIDERATION
2.1Screw Jack: It is a jack which is operated by turning a lead screw. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles. A screw jack consists of a strong screw shaft which extends vertically from a heavy supporting base, and is turned with a bar or lever. On the upper end of the screw is a bearing surface which supports the load. When the screw is rotated it extends, lifting the load.
2.2 Gear Mechanism: The Gear mechanism plays a vital role in transmitting the power/torque from Motor to the Screw Jack to perform this The Pinion and Gear have different number of teeth giving a gear ratio of 1.5 and reducing the speed of the motor in the same ratio.
2.3 Motor: The motor required in this project is not easy to find as it should have the
capability to run from a car battery producing 12V and at the same time producing huge
Amount of Torque so as to lift the vehicle with low r.p.m. of 80-100. Also the motor
should be bipolar as it should be capable to lift as well as to put down the vehicle when
the whole work is performed
CONSTRUCTION DETAILS:-
WORKING PRINCIPLE:-
In Brief: When the worm shaft is rotated the lead screw rotates in the body of the screw jack at the same rate as the worm gear. The nut on the lead screw moves in a linear direction along the screw when fixed to a structure that prevents it from rotating with the screw. This design is also available with a "Safety Nut".
In Detail: When a screw jack unit is operated, the rotation of the worm shaft causes the worm gear to rotate. For rotating screw jacks the lead screw is fixed to the worm gear and they rotate at the same speed. As the worm gear turns, the friction forces on the screw thread act to turn the nut also. The greater the load on the screw jack unit, the greater the tendency of the nut to turn. It is obvious that if the nut turns with the screw, it will not raise the load. Therefore the nut needs to be fixed to a structure to prevent rotation. The restraining torque required for the structure, also known as the "lead screw key torque"
The input power to the screw jacks should not exceed the power rating shown in the specifications table. Maximum input speed in rpm (revolutions per minute) to a screw jacks worm shaft should not exceed 1800 rpm for E-Series and M-Series screw jacks; however the high performance S-Series screw jacks can operate at up to 3000 rpm. Power Jacks cannot accept responsibility for the overheating and rapid wear that may occur should these limits be exceeded. Power increases in direct proportion to the speed, and the motor size will be out of proportion to the screw jack model design rating should the speed become excessively high. When selecting the maximum permissible speed for a screw jack arrangement, always check to see that the power rating of the screw jack model is not exceeded.