Drive Systems

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Driving The Pump
Pumps may be driven directly by an electric motor or gas engine, or connected to a motor or engine by a gearbox or by a belt and pulley system. There is a trend in the industry toward direct drive mountings due to their economy and simplicity. Mounting a pump directly to a motor eliminates the expense to the manufacturer of purchasing and installing pulleys and belts or a reducing gearbox. Additionally, direct mounting eliminates some moving parts which can wear out and require replacement. More pumps are being made available which are designed for direct drive operation at common motor and engine speeds. However, the faster a pump operates, the more rapidly wear will result.

Direct Drive Mounting

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For a direct mounting the pump drive shaft can be designed so that it encloses the motor shaft (a hollow shaft pump) or the pump can have a solid shaft that connects to the motor shaft with a coupling device. The key slot on the pump shaft is used in this type of mounting.

Many pump manufacturers today are marketing pumps that are already mounted on an electric motor or gasoline engine. The electric motors are generally “C-face” motors. They have a flat shaft side that allows very close mounting. NEMA standards dictate the location of the bolt holes on the motor’s mounting face. Pump manufacturers simply match their pump design to the holes for a specific NEMA standard C-face motor.

Reduction Gears

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In many instances, in order for a pump to be powered by a gasoline or diesel engine, a reduction gear assembly is required. This is necessary because the engine rpm differs from the pump’s rpm requirements. Reduction gear assemblies are available in a number of ratios. As in automobiles, lower gearing gives less speed and more power or torque. Additionally, a reduction gear assembly can allow the pump to operate at a lower speed, thus reducing engine wear and increasing pump life.

Belt Drive

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A belt and pulley system can be used to drive a pump. This works like reduction gears. The larger the pulley on the pump in relation to the pulley on the motor, the slower the pump action and the lower the pump’s operating rpm. The smaller the pulley, the higher the rpm figure.

Electric Motor Speeds
In the case of pumps mounted directly to an electric motor, it is important that the rpm ratings of the pump and motor match. Consequently, since electric motors generally are capable of driving a pump at 1750 rpm or, for some motors, double that or 3500 rpm, most pumps intended for direct mounting are designed to operate at one or the other of those speeds. The 3500 rpm, most pumps intended for direct mounting are designed to operate at one or the other of those speeds. The 3500 rpm pumps can also be used with 3600 rpm gasoline engines.

Actually, the standard electric motor speeds are 1800 and 3600 rpm but slippage reduces the speed at which it actually drives a pump at 1725 or 1750 rpm. Additionally, slippage makes it possible for the 3600 rpm gasoline engine to directly drive a 3450 rpm pump. In most cases a lower pump speed, however, means a reduction in pump wear.

Changing Pump Speed
Pumps can be operated at other than their rated or maximum volume by simply reducing the speed of operation or rpm. A simple formula can be used to determine the rpm requirement for a desired volume. Multiply the rated rpm by the desired gpm and divide by the rated gpm. For example, if you wished to operate a 1752 rpm pump with a maximum flow of 2.2 gpm at only 1.5 gpm you would determine the required rpm by multiplying 1752 by 1.5 for a result of 2587.5 and then divide by 2.2 to determine that the ump should be operated at 1176 rpm to produce a 1.5 gpm flow.

Introduction To Drive Belts And Pulleys
Belts and pulleys may be used to reduce pump pulley belt drive speed below the motor speed. To size the pump pulley for a specified pump speed when the motor pulley size is known, the following formula is used. Motor speed in rpm divided by desired pump speed in rpm is equal to the pump pulley diameter divided by the motor pulley diameter. In most cases the diameter of the motor pulley will be 3.4 inches.

Calculating Pulley Size
If a 1725 rpm motor is to be used to drive a pump at 950 rpm, the pump pulley size would be determined as follows:

1725 + pump pulley diameter or 1725 x 3.4 = 6.2 inches
950             3.4 inches                  950

Installing Pulleys
Once the pulley size combination has been determined, the proper pulley or bushing and sheave combination should be installed on the pump shaft. Place the sheave or pulley over the pump shaft with the taper facing inward towards the pump. The slightly tapered bushing fits over the shaft and inside the sheave. These should be tapped into place. The key way on the shaft and bushing must be lined up with the key inserted. There are four bolt holes on the bushing. Two of these holes are threaded and the other two are smooth bore. When installing the bushing, bolts should be set in the two bushing holes which are smooth bore and match up with the sheave holes. Tightening these bolts not only draws the bushing into the sheave but pinches the split and tapered bushing against the shaft. This method securely holds the sheave or pulley assembly in place.

Dismounting The Pulley
When the pulley assembly is to be removed from the shaft, the bolts are removed from the two mounting holes and inserted in the other two (threaded) holes in the bushing. This design is essentially an integrated puller.

One Piece Pulleys
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In some cases pulleys come as one piece without a bushing. These pulleys are simply pressed firmly on the shaft. In most cases where this type of pulley is used there is a provision for a retainer bolt to be screwed into the shaft.

Aligning The Pulleys
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Once the pulleys are mounted on the shafts the pump should be mounted so that the belt will turn in a straight line rather than at an angle between the two pulleys. To determine if the pulleys are properly lined up, place a straight edge across the outside of the two pulleys. The straight edge should be flat against the outside ends of both pulleys.

Installing The Belt
The belt or belts should be installed. There should be installed. There should be slots either in the pump mounting rails or plate or in the frame to allow for adjusting belt tension. Simply loosen the mounting bolts and slide the pump until the belt is properly tightened. If the belt is too tight bearing wear will result and if the belt is too loose, it will slip.

Adjusting Belt Tension

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Belt tension may be tested by pressing against the top of the belt midway between the two pulleys. The proper deflection is determined by the distance between the centers of the two shafts. To determine proper deflection simply multiply the distance between the center points of the two shafts by .016. If the distance is 18 inches then the proper deflection is 18 x .016 or .288 inches. This rounds out to ¼ inch, making a belt deflection of ¼ inch midway between the two pulleys an indication of proper belt tension for this configuration.

V-Belts
Most belts used to drive pumps are v-shaped belts called v-belts. For more heavy duty applications, heavier duty belts are required. Larger pumps should generally be driven by double v-belts. This means the drive will consist of two parallel pulley assemblies, each with its own belt.

Other Applications
Belts may be used for other purposes than driving the pump motor. In many European design boiler systems, the boiler fan or blower is driven by a belt turned by the pump motor. This belt is usually driven by a pulley on the reverse end of the motor from the end where the direct drive pump is mounted. Since not much power is required to drive the fan (compared to the power required to drive the pump), this belt is thinner than that used to drive the pump. In gas engine hot water cleaners, a generator may be belt-driven from the gas engine to generate electric power for the burner motor and ignition.

NOTE: When replacing dual or multiple belts, care must be exercised in selecting replacement belts. There is an identifying number on each belt which is simply a manufacturer’s identification or part number. There is also another important number which is a casting number or mold number. These numbers must match for both belts. If not, difficulty in achieving even belt tension may result.

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