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Design of a Worm Gear Term Paper


A worm gear refers to a type of gear whereby a worm gear connects with a worm in order to allow movements of a machine. It was invented in the 1st century by Heron from Alexandria. A version of the worm gear was invented in India around the 13th/ 14th century and was commonly referred to as a changing double roller was mainly used in the manufacturing cotton gins. The worm gear has shafts that are not parallel but do not intersect at all. The worm resembles a screw and the gear resembles a spur gear. This project was designed by the team in order to come up with a device that is operated using a worm gear set with specific qualities in order to be better in terms of functionality.

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There are 3 main types of worm gears that can be used in worm drives. The first type of worm gear is the non- throated. Based on its name, this gear lacks the groove usually located on the wheel or the worm’s circumference. This type is flat with no deflation. The second type of worm gears is the single – throated worm gears. This type is whereby only the worm wheel has the groove on its circumference. The last type is the double throated worm wheel which is the type of worm wheel that contains grooves in both the worm wheel and the worm. The double throated worm wheel can be able to accommodate heavier goods than the other two types.

There are also two main classifications of worm gears. The first one is the enveloping worm gear which contains one or more than one tooth and is thick in the middle and thinner in both ends. The second one is the double enveloping worm gear which is made up of worms that are connected with worm gears using enveloping worms and are also termed as globoidal worm gears. The team chose a winch as the worm gear device for this project.

A winch, schematically showed below, is operated by a worm-worm gear set. The input torque to the worm is provided by an electric motor that rotates at 1,725 rpm. The expected speed of the worm gear is between 30 and 40 rpm. The peak torque requirement for the winch is about 4,000 lbf·in, the operating ambient temperature is 120 °F, and the average output power doesn’t have to exceed 2.0 hp. Knowing that the winch drum radius is 8 in, and that the winch operates for 10,000 hours, the team performed the following calculations:

⦁ The team designed the worm-worm gear set that meets the requirements for the operation of the winch. These requirements are performed force, bending and wear analyses for the gear set. To do this, design factor of 1.1 was assumed for the gear teeth and ensured that the mesh is self-locking. Using a gear manufacturer’s catalog,a readily available gear mesh that meets the design requirements was selected.The mesh selected was supposed to function for 10,000 times.

⦁ The bearings required to support the shaft holding the worm-gear was selected.

V1 = Vm = (πd1n1/60000)
= π x 50 x 1725 / 60000 = 4.51 m/s
n2 = n1/ i = {n1 (Z2/Z1)}
= 1725 / (32/2) = 107.8 rpm
V2 = (πd2n2/60000)
= πx128x107.8/60000 =0.72m/s
VS = V1 /cos λ = 4.51 / cos9.09◦ = 4.567 m/s
Worm, Φn = tan-1 (tanφ1cos ψ) = tan-1(tan20◦ cos 9.09◦) = 19.64◦

3). The shaft for stress and deflection constraints that will support the worm gear was designed. The worm gear had shafts that were not parallel but did also not intersect at all.
A computer code that allows the design of the worm-worm gear was set based on the given inputs, meeting the imposed bending and wear requirements for the gear set. In this project the operating system that is utilized in the development of the design was the PC (MS- DOS) program. The design must also be able to allow the various hardware functionality by being valid to any hardware that is compatible to IBM that has a favorable operating system.

A computer code that calculates the deflections on the shaft was designed to support the worm gear and the drum. The shaft design was meant to deal with fatigue problems as well as the device’s rigidity.
In order to select the bearings and the attachments of the shaft to the worn gear, a few steps were analyzed. First, the torque required by the winch at the output shaft was determined. The gear acts as a source of power transmission to the entire device. The amount of torque produced by the gear determines the speed of the device as well as the amount of work that the device can be able to handle.

Secondly, the motor types and the effectiveness of the gears were analyzed in order to increase the efficiency of the energy being produced. The team rejected the use of the permanent split capacitor (PMC) and instead replaced it with the permanent manet dc (PMDC) so as to enjoy the advantages that come with gear motors.

In addition, the design needed to be of a good size, for instance not so big or small. This is because an undersized worm gear can lead to the failure of the device functionality while an oversized gear leads to the device being less efficient. Moreover, the power input versus the power output had to be of a certain ratio in order to allow proper efficiency of the worm gear as well as the connection of the shaft. The input is the power that enables the gear to work while the output is the torque produced by the load and the speed of the gearbox.

Furthermore, the maintenance and management of the design had to be followed by the user depending on the conditions set by the manufacturer. On several occasions, manufactureres design devices that have applications in web softwares whereby the user can be able to select the parameters required. This web software is responsible for making calculations as well as units to be selected. Also, the worm gear failures should be carefully analyzed. These include:

⦁ The shafts are easily fatigued especially when there is excessive loading of goods. This can lead to destruction of the bearing support systems.
⦁ Large forces can cause the bearings to overwork and therefore destroy the gears.
⦁ Gears that are used in high temperatures have high chances of facing condensation.
⦁ Lubrication should made only during operating conditions. However, lubrication done at all times is more expensive than that doe during extreme environmental conditions.
⦁ The quality of the product is limited when the mounting of motors is unstable. This instability causes flexing and misalignment.
⦁ Too much temperature causes overheating of the gear which causes malfunctioning of the device.
Apart from the PMDC, the integrated ac motor is also efficient when it comes to optimal performance of the worm gears design. This is because the motor has a low inertia rotor which has the same characteristics as the gear unit. This equality is responsible for the dynamic performance of the gear in terms of braking and acceleration of the device.

The integrated ac motors also function in a better way with inverters such as the Variable Frequency Drive (VFD). Together, they work using the closed- loop feedback in order to allow the application of point to pot positioning and also indexing. Apart from that, drives that use electricity in order to function enable proper control and speed of the device, torques that can be adjusted and finally protection of the device against overloading.

Most devices that use a combination of motors and gears experience a high level of performance and gains. Finally, the team looked for more information in the internet so as not to miss important features of the design. The internet provided resources for both manufacturers’ websites as well as industries’ associations for the qualifications of the best worm gears to be used.

American Gear Manufacturers Association (AGMA)
The team selected the part numbers of the worm gears to be between 2″ and 48″. The worn gears are manufactured by several manufacturers. The team was able to select a few manufacturers for their design. These manufacturers included the Philadelphia Gear company, the Link Belt Company, Falk Company and the Westinghouse Company.

Link Belt – this company is responsible for replacing worm gears that were manufactured as from 1940 – 1976

Falk – this company is responsible for the replacement of worm gears that were made as from 1973 – 1988

Westinghouse – this company deals with the replacement of models that were manufactured as from 1963 – 1970

Philadelphia Gear – this company replaces worm gears manufactured from 1985 – 2002
A part drawing for the designed shaft including appropriate GD&T specifications is as shown below whereby the bearings and the shafts were effectively placed.

During the process of designing the worm gear, one of the assumptions made is that the worn gears transmit motion at an angle of 90◦. This is because the worm is in a horizontal position while the worm gear is in a vertical position and the motion is caused by the rotational movement of the gear which in turn causes the worm to rotate. The team managed to come up with the input torque to the worm which was provided by an electric motor that rotated at 1,725 rpm. The team also made the speed of the worm gear to be between 30 and 40 rpm. The peak torque requirement for the winch was about 4,000 lbf·in, the operating ambient temperature was 120 °F, and the average output power was less than 2.0 hp. The winch drum radius was 8 in, and that the winch operated for 10,000 hours, therefore the team came up with a bearing angle of 19.64◦.

Work’s Cited
Jones, Franklin D, and Henry H Ryffel. Gear Design Simplified. Industrial Press, 1961.
Worm Gearing. Industrial Press, 1910.

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