RECURSOS ASIGNADOS AL IFAM 2009 AL 2013

Chain Drive

Chain drives are often used in the same manner as synchronous belts and provide positive synchronization between transmission shafts, especially when large amounts of torque are required (Figure 2-53).

The stretching action of belt systems is not a problem for a chain drive. However, depending on the number of teeth in the drive gear sprockets, slight pulsations (whipping action) could occur in the drive gear sprockets. This can cause a noise or vibration problem if the pulsations are great enough.

Couplings, Gearboxes, and Speed Reducers

In many cases, some positive method of connecting two shafts together is required when the shafts operate along the same centerline. The two most common types of couplings are rigid and flexible couplings.

The two most common types of rigid coupling are the flange and the sleeve type.

Figure 2-53. Synchronous belts system

Synchronous Belt Pulleys Cross-Sectional View Chain Drive Cross-Sectional View

Flange Coupling

As the name would imply, the flange type uses two metallic flanges slipped over the ends of the two shafts and bolted together. Each individual flange is either pressure fit over the shaft keyway, or locked onto the shaft by some other method such as a set screw (Figure 2-54).

Flexible Couplings

The function of the flexible coupling is the same as the rigid coupling. The difference between the two is the flexing or twisting capability of the cou- pling parts (within limits).

A certain amount of flexing is expected with this type of coupling. Flexing may be caused by: misalignment of the shafts, shaft end movement due to insufficient motor and machine mounting, or simple vibration between the connected devices.

The flexible part of the coupling could be considered a “mechanical fuse.” If the flexible part fails, it is a sign of mechanical drive trouble in the sys- tem. The mechanical system should be thoroughly checked out before the flexible part is replaced, which is the same for an electrical system replace- ment. We will take a brief look at the two types of flexible couplings: mechanically flexible and elastically flexible.

Mechanically Flexible

These types of couplings obtain their flexibility from the rolling or sliding of the mating parts. As expected, these parts do require some type of peri- odic lubrication. Typical examples are gear, chain, and sprocket and disc types, shown in Figure 2-55.

Elastically Flexible

This type of coupling (sometimes referred to as elastomeric) obtains its flexibility from the stretching or compressing of a material such as rubber or plastic. The sliding or flexing that takes place is minimal, and lubrication is not required. These types of couplings come in designs such as jaw, clamped, or unclamped donut, and the tire (Figure 2-56).

Figure 2-54. Flange and sleeve-type couplings

Flange Type

Sleeve Type

Set Screws Fastening Bolts

Chapter 2: Review of Basic Principles — Mechanical Devices 63

Gearboxes and Speed Reducers

Gearboxes and speed reducers transmit mechanical power from the motor shaft to the driven load. A simple speed reducer contains two different sized gears, called spur gears. Spur gears are straight teeth cut parallel to the axis of rotation. Speed reducers may have more than two gears, which provide several fixed speed outputs. If that is the case, some type of clutch arrangement will exist to change gears. Figure 2-57 shows the gear rela- tionship within a speed-reducer case.

As seen in Figure 2-57, this type of device provides an efficient means of transmitting positive speed, direction, and torque. In many cases, this device will change speed, with a corresponding change in torque or output

Figure 2-55. Mechanically flexible couplings

Figure 2-56. Elastically flexible couplings

Disc on each shaft is bolted to common center bracket Jaw Elastic Material Elastic Material Donut (Unclamped) (Clamped) Tire

direction. The gear reducer acts as a torque amplifier, increasing the torque output by a factor proportional to the ratio, less an efficiency factor.

As seen in Figure 2-57, if a 1150-rpm motor delivers 4.5 lb-ft of torque to the input shaft, then 153 lb-ft of torque is present at the output shaft (given the efficiency of 85%). The formula used is lb-ft × reducer ratio × efficiency of the speed reducer. Therefore, 4.5 × 40 × 0.85 = 153 lb-ft of output torque.

To change the output speed by more than one fixed ratio, more than two gears are required. The same is true if the direction of rotation of the out- put shaft needs to be identical to that of the input shaft. The type of gear used depends on the application (i.e., horizontal axis of rotation, vertical to horizontal axis of rotation, etc.). Figure 2-58 shows several types of gears used in common speed reducers.

As shown in Figure 2-58, the speed reducer is operating at 85% efficiency. A brief look at mechanical efficiency would be helpful in understanding the actual output of the device. As previously stated, efficiency is the ratio of output power to input power and is expressed in a percent. Therefore the formula for efficiency would be:

When considering the efficiency of a belt type of device, the more friction available, the less slippage that occurs, and the higher the efficiency. The output shaft turns simultaneously with the input shaft.

In belt systems, several factors aid in decreasing total system efficiency: (1) Losses that occur due to friction of the rubber and cords as the belt

stretches and flexes. (2) Friction of the belt as it enters and leaves the pul-

Figure 2-57. Speed-reducer characteristics

Input Shaft Output Shaft Speed Reducer 1 HP 1150 RPM 4.5 lb-ft 1 HP 28.75 RPM 153 lb-ft 40:1 Ratio 85% Efficiency

Speed Reducer Gears

Output Shaft Rotation is opposite Input Shaft Rotation input 100 output (%) Efficiency = ×

Chapter 2: Review of Basic Principles — Chapter Review 65

ley or sheave. (3) Bearing friction caused by excessive tension on the pul- ley (or sheave) causing a high level of drag in the bearings.

Efficiencies of belt transmission systems can be as high as 90–98%. When proper maintenance and belt tension is maintained, these efficiencies can be sustained throughout much of the system’s usable life.

Chapter Review

Electrical principles play a major role in understanding how electrical power is modified and controlled in a drive unit. All electrical circuits have three main factors: current, voltage, and resistance. Current is actually the flow of electrons that cause the work to happen in a circuit. Resistance is the opposition to current flow, which is present in any circuit (all matter in almost all modern electrical circuits have some resistance). Voltage is the electrical pressure that tries to overcome resistance.

Two types of voltage are available: direct current and alternating current. Direct current is typically found in battery circuits and its output value does not fluctuate, until the battery goes dead. Alternating current is typi- cally found as a household and industrial power source. The output value fluctuates depending on the time base of transmission (positive to negative to positive output). Frequency is the number of times complete cycles are seen in a power system (e.g., 60 Hz). Two types of AC are available: single phase and three phase.

Electromagnetism is the ability to produce a magnetic field through the use of a voltage and a coil. Electromagnetism plays an important role in the transmission of voltage through inductors, coils, and motor windings.

Figure 2-58. Speed reducer gear types

Helical Gears

Bevel Gears Worm Gears

Spur Gears

Devices using this principle are: inductors, relays, contactors, and trans- formers. Inductance is the ability to block AC voltage and allow DC to flow. Capacitance is the ability to block DC and allow AC to flow. Both inductance and capacitance play a role in the filter circuit in an AC drive. Capacitance in an inductive circuit (motors) tend to improve the power factor of the entire system. The power factor is the measure of the efficient use of the current waveform and is stated as a ratio between the utility generated voltage and current waveform.

Semiconductors are a mix between conductors and insulators; they need an electrical push to drive the device to conduct current. Typical semicon- ductor devices include diodes (which allow current flow in one direction), thyristors (SCRs that conduct current only when triggered on), GTOs (which can be latched on or off depending on the polarity of the gate sig- nal), transistors (which control the amount of current flow determined by the amount of trigger signal), and IGBTs (specialized transistors that have extremely high speed switch on and off times).

There are three basic types of mechanical loads that are encountered by any AC or DC drive-system: constant torque (e.g., conveyors), variable torque (e.g., centrifugal fans/pumps), and constant horsepower (e.g., machine tools).

Speed, torque, and horsepower play a major role in the operation of any application. Speed affects how much horsepower is required to perform the function—faster speed requires more horsepower. Torque is a turning effort and defines the ability of a system to start and keep moving at a specified rate. Inertia (WK2 or WR2) is the measurement of an object’s resistance to change in speed. This measurement is needed to determine the acceleration time available from a drive system.

Gears, belts, pulleys (sheaves), chains, and sprockets all work to allow a smooth transmission of mechanical power, and in some cases, change speed and direction. Types of belts and pulleys include flat and v-belts, synchronous belts, and chains and sprockets.

Couplings, gearboxes, and speed reducers offer a positive connection point between the motor and application. Couplings are available in the follow- ing designs: flange, sleeve, and flexible (mechanically and elastically).

Speed reducers offer an effective means of changing speed delivered to the application, as well as the torque developed by the motor. If the speed is reduced at the output of the reducer, torque is increased by approximately the same amount. Speed reducers are available in various types such as helical gears, spur gears, worm gears, and bevel gears.

Chapter 2: Review of Basic Principles — Check Your Knowledge 67

Check Your Knowledge

1. For current to flow, what must be added to overcome resistance? 2. What devices are used to measure current and voltage, respectively? 3. What is DC?

4. What is magnetic flux? 5. What is electromagnetism? 6. What does 60 Hz refer to?

7. Capacitance has the ability to block ____ and let ____ pass. 8. Inductance has the ability to block _____ and let _____pass. 9. Give an example as to where inductance may be helpful. 10. What is power factor?

11. What is one benefit of placing an inductor ahead of the drive? 12. What two circuits make up a relay circuit?

13. How does a transformer conduct output voltage without any physical connection?

14. Indicate a use for capacitors in the DC bus circuit of an AC drive. 15. Explain the operation of a diode.

16. How does an SCR operate? 17. How does a GTO operate?

18. What is a Darlington bipolar transistor?

19. How do IGBTs differ from the common transistor? 20. What are the characteristics of a constant torque load?

21. Indicate two applications that fall into the constant horsepower category. 22. How does an AC drive save the operator money when connected to a

centrifugal fan?

23. What is the definition of torque? 24. What is the definition of horsepower? 25. What is the definition of inertia?

26. If a speed reducer has a 10:1 ratio, with 3 lb-ft and 1200-rpm input shaft, what speed and torque would the output shaft be (assume 85% effi- ciency)?