Mechanics and Manufacturing Methods: part 1

1. MOTOR MANUFACTURING PROCESS FLOW

The basic manufacturing processes for electric ac and dc motors are shown in this section. Within each process, there are significant variables, each depending upon the manufacturing equipment, size and variety of the parts, electrical efficiency requirements, and economics. Each one of these process variables is described in the following text.

1.1 AC Motor Manufacturing Process Flow

ill1 illustrates a basic ac motor manufacturing process flow. The first step is producing laminations. These laminations are separated into rotors and stators. The stator laminations, shown in ill.2, are then stacked into a core, and copper and /or aluminum wire is wound into the core, producing a wound stator core. An outer housing of some type is produced, and that's then wrapped around the wound stator core, making a wound stator assembly. The wound stator assembly is then sent to motor assembly.

The rotor laminations in ill.3 are also stacked and then aluminum diecast into a rotor casting, shown in ill.4. A shaft is then produced, and this is assembled into the rotor, making it a rotor assembly, shown in ill. 5. The rotor assembly is sent to motor assembly.

Two end frames are produced and sent to motor assembly.

At the final operation, the wound stator assembly, rotor assembly, two end frames, and miscellaneous parts are assembled into a complete motor. The motor is then tested, painted, and packed for shipment.

ill. 1 Ac motor manufacturing process flow.

ill. 2 Stator laminations.

1.2 DC Motor Manufacturing Process Flow

The basic dc motor manufacturing process is illustrated in ill. 6. Like ac motors, the first step is producing laminations for the pole piece and armature.

The pole-piece lamination is stacked with several other components into a pole piece assembly. The pole piece on dc motors may be of solid steel, as shown in ill. 7. A housing is produced, and when the pole pieces are inserted, it becomes a frame and field assembly, shown in ill. 8. This frame and field assembly is then sent to motor assembly.

ill. 3 Rotor laminations.

ill. 4 Rotor casting.

ill. 5 Rotor assembly.

Brushes, with other components, are assembled into a brush assembly, as shown in ill. 9, and this is then assembled on the frame and field assembly.

The armature lamination is stacked into a core, which is then assembled onto a shaft, and copper wire is inserted or wound onto the core. The coils may be connected to the commutator as they are wound, as in ill. 10, or connected after the coils are inserted into the core and shaft assembly, as in ill. 11. This is a completed armature assembly which then goes to final motor assembly.

The frame and field assembly, armature assembly, and miscellaneous parts are then assembled into a complete motor, as shown in ill. 12. The motor is then tested, painted, and packed for shipment.

3.2 END FRAME MANUFACTURING

The basic purpose of an end frame, sometimes denoted an end bell, end shield, or bracket, is to contain the shaft bearings and support the rotor assembly. It will also act as a heat transfer device. On open motors, the end frame will have slots for air to pass. On enclosed motors, the end frames will be solid, with no openings. A variety of end frames are shown in Figs. 13, 14, and 15.

Like housings, end frames come in cast-iron, steel, zinc, or aluminum castings.

Cast-iron castings are usually found on motors of 3 hp and larger. The service application is in the industrial market where severe conditions may exist. Materials are usually of about 30,000 lb/in 2 tensile strength and are free machining. The typical sequence of operations is a two-machine cell-a computer numerically controlled (CNC) machine prepares the bearing bore and end frame diameter, and a manual drill is used to prepare the holes for the housing attachment.

The steel material is usually SAE 1010 to 1020. This type of end frame may be found on all types and sizes of motors. A coil is processed through a stamping press, and each part is drawn into form as a stamping. This is usually a progressive die operation.

A self-aligning bearing is installed and lubricant is applied. Then the bearing is sized for the only machining process.

Zinc or aluminum end frames are found on most motor types and sizes and generally are castings. End frames are usually cast in a horizontal die caster. Because of its density, zinc is usually limited to end frames for motors 3” in diameter or less. If the parts are small enough, more than one part is made at one time. This depends on the part and machine sizes. Also, on motors above 1/.4 hp, a steel bearing insert is usually diecast in the part. Following the die casting and part cooling, the part is trimmed. Many manufacturers have installed robots for this operation because of the heat and environmental conditions.

ill. 6 DC motor manufacturing process flow.

ill. 7 Solid steel pole piece.

ill. 8 Frame and field assembly.

ill. 9 Brush assembly.

ill. 10 Winding a commutator.

ill. 11 Core and shaft assembly.

ill. 12 Complete motor.

ill. 13 End frame.

ill. 14 End frame.

ill. 15 End frame.

The bearing bore and housing end frame diameter of the end frame are then machined. This is done on either a CNC lathe or a special automatic machine, depending on size and volume.

Some very small motors use an oil-soaked wick, as seen in ill. 14, for lubrication.

This is inserted after machining.

3 HOUSING MATERIALS AND MANUFACTURING PROCESSES

Housings, also known as frames, come in all types of materials and configurations.

Basically, the housings are made in the same way for both ac and dc motors. The basic purpose of the housing is to cover the stator or pole-piece assembly, provide heat transfer and protection, provide a location for mounting the end frames, and serve as an attachment for other components, such as outlet boxes and lifting hooks.

3.1 Materials and Configurations

The housings come in cast iron; in rolled, wrapped, and tube steel; and in both cast and extruded tube aluminum.

Cast Iron. Castings are usually found on motors of 3 hp and larger. The service application is in the industrial market where severe conditions may exist. Materials are usually of about 30,000 lb/in 2 tensile strength and are free machining.

In most cases, the mounting feet are cast as part of the housing.

Steel. As mentioned, steel housings come in several configurations-rolled, wrapped, and tube. The material is usually SAE 1010 to 1020. This type of housing may be found on all types and sizes of motors.

Aluminum. This material is also found on most motor types and sizes. The cast housings may be produced for a size as large as NEMA 360 but are usually not found on motors rated below 3 hp.

The tubing may be found on the smallest motors up to about 25 hp. The material is usually SAE 6061.

3.2 Manufacturing Processes

Cast Iron. The typical sequence of a cast iron operation is as follows:

1. Machine and drill the mounting feet to be used as a locator for further machining operations.

2. Bore the inner diameter (ID).

Turn the end frame registers (optional-sometimes done as a wound stator assembly).

4. Drill and tap for the end frame attachment.

5. Mill for the outlet box attachment.

These machining operations can be completed on either manual machines or CNC machining centers. Usually machine-tool cells are incorporated.

Rolled Steel. A coil is processed through a stamping press and the shape is a flat form. This piece is then formed around a mandrel and welded. In some cases, the weld is a straight butt weld. In other cases, the rolled end attachment is interlocked mechanically with several weld beads.

The housing is then machine-faced to length. Next, a stamped mounting base is welded to the housing.

There are both highly automated and semi-manual machines for this process.

Wrapped Steel. The manufacturing processes are the same as for a rolled housing, except that the stator core is used as the mandrel.

Tube Steel. A drawn-over-mandrel (DOM) tube or a hot-rolled seamless tube is processed in the following manner.

DOM. Cut to length, machine end frame diameter (optional-may be done as a wound stator assembly), and weld mounting feet.

Seamless tubing. Cut to length, machine end frame diameter (optional-may be done as a wound stator assembly), and weld mounting feet. Depending on the condition of the bore, it may have to be machined.

Aluminum Castings. Most aluminum castings are produced as a complete housing with mounting feet. These are machined like cast iron and with the same type of equipment. Some, however, are cast over a stator core. This process requires machining like cast iron, except that the bore isn't machined.

Aluminum Tubing. The material is cut to length. Sometimes the end housing diameter is machined prior to stator core assembly. The mounting feet are then welded or screwed to the housing.

4 SHAFT MATERIALS AND MACHINING

4.1 Shaft Materials

Most motor manufacturers use SAE 1045 in either cold-rolled or hot-rolled steel (CRS or HRS). Other materials include sulfurized SAE 1117, SAE 1137, SAE 1144, hot-rolled SAE 1035, and cold-rolled SAE 1018. A ground stock of any material is used on special CNC Swiss turning machines.

Generally, the cold-rolled and sulfurized steels will cost about 15 percent more than HRS and will machine better. Machining trials need to be performed in order to justify the extra cost. Since all shaft-turning machines perform differently, there is no established material or machining practice.

Obviously, the hot-rolled plain carbon steel, on a cost-per-pound basis, is cheaper than cold-rolled sulfurized steel. But there are tradeoffs. The hot-rolled material has to be sized larger than cold-rolled because of the lack of outer diameter (OD) control in the rolling process. A manufacturer has to evaluate whether the larger-size and lower-material-cost hot-rolled bar stock is more or less costly than cold-rolled bar stock. Also, the hot-rolled material, by the very nature of its processing, has hard and soft spots, residual stresses, voids, and other material deficiencies, making machining more difficult. Again, machine trials need to be conducted to obtain the best cost option between CRS, HRS, nonsulfurized, and sulfurized materials.

Because of the difficulties with HRS, most motor manufacturers will use sulfurized CRS.

4.2 Machining Operations

Most manufacturers saw, shear, or turn the shaft length off the original bar stock.

Sawing is done with a band saw, machine back saw, or rotary saw, and the material is cut either as a separate piece or in bundles.

One process, to eliminate the saw-cut kerf material, is a shear cutoff process. It is very fast and noise has been eliminated. However, this meets with mixed results. In the shearing process, the end of the bar is deformed-the top of it's formed down-ward and the bottom has a burr, as illustrated in ill. 16. This deformation has to be removed in the face-and-center operation, which is sometimes difficult and causes excess tool wear.

ill. 16 Shear cut-off process.

The third option is to cut off the shaft bar in a bar-turning machine. The bar-turning machine will complete the shaft diameter machining, and as a last operation a cutoff tool will remove the shaft from the bar.

Nearly all shafts for motors larger than 1/4 hp have to be faced and centered for future machining operations. This operation is usually completed on one machine with a special face-and-center tool.

Both ends of the shaft are centered to provide a tool location in the lathe turning operation and in balancing as a rotor assembly. Facing is also done in order to pro-vide a more precise length in turning and when face drivers are used in the turning operation.

Many motor manufacturers combine the bar cutoff and face-and-center operations. Most motor manufacturers now use CNC turning machines because of their quick setup changeover capabilities, capability of completing a shaft in one operation, and ability to precisely turn a diameter to 0.0005-in tolerance and meet the surface finish requirements.

On motors greater than 1/2 hp, the bearing journal tolerances are generally 0.0005 in or higher. The ability to turn bearing journal diameters to a 0.0005-in tolerance has eliminated the subsequent grinding operation.

Some motor manufacturers that produce shafts larger than 2 in (3 hp and up) use a retractable jaw chuck in combination with a face driver, rather than a face driver alone, in order to maximize the machine horsepower yet provide the necessary precision.

This type of chuck also works well on hot-rolled bar steel because it provides better clamping of the bar than do face drivers. The chuck jaws retract under the semi-finish turning operation to allow turning under the jaws. Then the CNC machine completes the finish turning to size using the face drivers.

Most motor manufacturers combine keyway milling (on a manual machine) with the CNC lathe in a one-person cell.

Some motor manufacturers started incorporating CNC Swiss turning machines when they became available in the mid-1980s. These machines can machine a bar up to about 2-in. in diameter and hold tolerances to 0.0003 in. They incorporate complete turning, including keyway milling, plus other special features such as threading and grooving. The process helps assist flexibility in short runs and in completing parts of extensive complexity. However, these machines require centerless ground stock, which is more expensive than CRS or HRS. Again, the economics will dictate the method of operation and equipment.

If the bearing journals require a size tolerance better than 0.0005 in, a separate grinding operation is usually required.

Other machining options are the use of manual multispindle machines for cutoff and turning and the use of grinders for grinding bearing journals and seal diameters.

This option is usually used for shaft diameters 1 in and smaller. A high-volume option for 1-in and smaller shafts is a dedicated transfer line which uses ground bar stock.

Some motor manufacturers, particularly those that produce sizes of 5 hp and up, finish-machine the bearing journals and rotor diameter as a rotor assembly. This operation produces the best possible concentricity between the bearing journals and rotor diameter.

Few motor manufacturers have had success with postprocess gauging with feed-back size compensation in the bearing journal finish-machining operations. How-ever, this is expensive and isn't always accurate because the part has to be clean.

Some people believe that once a shaft is removed from the turning operation, one can't use the centers for location in future operations. However, the method used is to set up a finished shaft (with or without rotor) in a lathe to indicate the drive end and both journals. If the output end is within 0.0005 in of true inner radius (TIR) and both journals with respect to each other are within 0.003 in TIR, turn the rotor OD as is. If not, adjust centers to get the acceptable TIR.