A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear, and is also called a worm wheel). The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.
Like other gear arrangements, a worm drive can reduce rotational speed or allow higher torque to be transmitted. The image shows a section of a gear box with a worm gear being driven by a worm. A worm is an example of a screw, one of the six simple machines
Gear box is required for:-
1. House and hold the gears in place
2. Protect the gear mechanism from any mishap.
3. Protection from dust etc
4. To keep the lubrication
Gears are required for:-
1. To reduce / increase the rpm.
2. Change the direction of rotation (clockwise / anti clockwise.)
3. Shift the axis of rotation.(linear or angular)
Control over power output, by means of the throttle pedal, simply regulates the rate at which the engine is doing work: at very high speeds,the power output will be correspondingly high but the torque output can at the same time be significantly less than at considerably lower speeds. In other words, maximum torque may be available over only a very limited speed range. Consequently, one needs to be able to regulate both the power output and the speed range of the engine relative to the range of speeds over which the vehicle is at any given time likely to be required to operate. Only in this way can the torque at the wheels be balanced against demands for either a steady speed uphill or downhill, or on the level, or for acceleration or deceleration. A gearbox is necessary, therefore, so that the driver can regulate torque by selecting the appropriate speed range or, in other words, the vehicle speed at which maximum torque is obtainable.
When a vehicle is moving at a uniform speed, the driving force, or tractive effort, at the wheels must be such as to exactly balance the sum of three categories of variable forces tending to oppose the motion. If it is greater, the car will accelerate, and if it is smaller, it will decelerate until a balance is obtained. Such a balance will be established eventually, because two of the forces vary with speed.
The three forces are:-
(1) aerodynamic, or air, resistance;
(2) gradient resistance, which can be either positive or negative;
(3) rolling resistance.
Gearbox selection will be done in two stages:
1) Selection of gearbox type
2) Determining the size of the selected gearbox type
Selection of Gearbox Type
The gearbox type suitable for the driven machine is determined during project phase. The technical staff designing the project decides on the type of gearbox according to his past experiences or examining applications done before.
For systems requiring less power, motor gears are the recommended solution. Motor gears are less expensive and are easily mounted. Shaft mounted gearboxes are also preferred due to same reasons.
Gearbox transmission ratios, powers and summary information on gear systems are given on Catalog.These are necessary in selection of a Gearbox.
Determining the size of the selected gearbox
Gearbox size can be determined with the help of the gearbox power tables. For each gearbox type a separate power table is laid out. The table shows the maximum power (kW) a given gearbox size can transmit according to the input speed and transmission ratio. The output torque the gearbox can transmit is also given in a separate table.
The power and torque values given in tables are nominal values. The power and torque needed for the driven machine should be less than the nominal power and torque the gearbox can transmit so that the gearbox can operate at a certain safety.
The power Nre, the gearbox should transmit, is determined by multiplying the power Nd needed for the driven machine by the operation safety factor. The nominal power of the selected gearbox should be equal to or more than the Nre value.
Please find below the method to calculate the operation safety factor.
Operation safety factor (F):
Coefficients determining the factor:
1) The coefficient for the driven machine, fd
2) The coefficient for the driving motor, fm
3) The coefficient for the daily operation time, ft
4) The coefficient for the start-up number, fs
Operation safety factor F is attained by multiplying the four coefficients:
F = fd . fm . ft . fs
The driven machine coefficient (fd)
This coefficient depends on the type of driven machine. Driven machines can be classified in four main groups according to their level of shock loading and moment of inertia.
Uniformly operating machines without shock loading: For machines with constant power demand, which operate without shock loading and without sudden moment increase, the coefficient fd is taken as 1.
Machines with medium shock loading: For machines which operate with medium shock loading and where load increases or decreases by at most 50%, the coefficient fd is taken as 1.5.
Machines with heavy shock loading and high moment of inertia: For machines with high inertia which operate with heavy shock loading and where load increases or decreases by at most 100%, the coefficient fd is taken as 2.
Machines with very heavy shock loading and very high moment of inertia: The coefficient fd is taken as 2.5-3 for this group.
The types of machines within these four main groups are given in operation safety factor table.
Driving motor coefficient ( fm )
This coefficient is determined according to the type of motor driving the system. There are three main groups:
1.Group: Electric motors (Asynchronous, synchronous, direct current motors), steam turbines, hydraulic motors. The fm for this group is taken as 1.
2.Group: Internal combustion, 4 - 6 cylinder engines (gasoline or diesel), water turbines. The fm for this group is taken as 1.25.
3.Group: Internal combustion, 1 - 3 cylinder engines (gasoline or diesel.) The fm for this group is taken as 1.5.
Daily operation time coefficient ( ft )
If the daily operation time is between 3 - 10 hours, the ft coefficient is taken as 1.
If the daily operation time is less than 3 hours, the ft coefficient is taken as 0.8.
If the daily operation time is between 10 - 24 hours, the ft coefficient is taken as 1.25.
Start-up number coefficient ( fs )
If the system makes at most five start-ups in an hour, the coefficient fs is taken as 1.
If the system makes more than five start-ups in an hour, the coefficient fs is taken between 1.25 – 2. In this case, special precautions might be needed. Connecting the gearbox to the motor with hydraulic or electro-mechanical coupling will reduce the effect of shocks during stop-starts.
An example for operation safety factor calculation
The safety factor (F) for a system operating 24 hours daily with an electric motor, making at most five start-ups in an hour, with moderate shocks:
F = fm . fd . ft . fs = 1 . 1.5 . 1.25 . 1 = 1.875.
F can be taken as 1.8 or 2.
Operation safety factors (F) are given in the table on page 19. In this table, driven machines are classified into four groups; without shock loading, medium shock loading, with heavy shock loading, and with very heavy shock loading and high moment of inertia. This classification is a guidance based on past experiences. Considering economical and safety factors together, the operation safety factor can be taken lower or higher than the value in the table.
Thermal Power (Nt)
This is the power the gearbox can transmit without heating up. Thermal power of gearbox depends on the size of external surface of the gearbox housing, ambient temperature, operation area (covered place or open air) and the operation time in an hour (FD).
Thermal power values each gearbox size can transmit are given in gearbox power tables. Thermal power values (Nt) given in tables are valid for gearboxes operating in a closed area, with 20 - 30C ambient temperature and for continuous operation (FD = 100%).
For 40C ambient temperature, 75% of the value given; for 50C ambient temperature, 60% of the value given in the table should be taken.
If the gearbox is operating 30 minutes in an hour (FD = 50%), Nt can be increased by 25%. For an operation of 15 minutes in an hour (FD = 25%), Nt can be increased by 50%.
If the gearbox is cooled with air or the gearbox oil is cooled, thermal power can be increased by 10% - 50% depending on the effect of the cooling.
Determination of gearbox size
Torque required for the driven machine is determined either by calculation or by past experience. Speed suitable for driven machine is again found either by experience or by trial and error method.
The power required to obtain the required torque for the driven machine at the selected speed is calculated in (kW) as:
Td (N.m) . nd (rpm)
Nd = ____________________
Provided that torque required for the driven machine stays constant, if the machine will operate at variable speed, power required should be calculated considering the highest speed.
Operation safety factor F can be taken from the table taking into consideration type of driven machine, motor and daily operation time.
The power gearbox should transmit (Nre) is obtained by multiplying power required for driven machine (Nd) and operation safety factor. The Nre value is the key for gearbox selection.
Nre = F . Nd
The power table for selected gearbox type shows the nominal power Nn (kW) a given gearbox size can transmit according to the input and output speed. The gearbox size having a nominal power greater than or equal to the Nre should be selected from the table.
The Nre value calculated should be less than the thermal power (Nt). ( Nre < Nt ).
If Nre is close to or greater than thermal power, gearbox should be forced cooled. If there is no cooling possibility, a greater size should be selected.
Hence, gearbox selection is completed.
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