Mechanics and Manufacturing Methods -- part 2a

3.5 SHAFT HARDENING

In many instances it's desirable to harden shaft materials. Harder shafts take longer to wear out than softer shafts. Shafts that are too hard become brittle and subject to fracturing. The exact hardness required depends on the intended use of the motor and the life required. Generally, when the shaft is used with a sleeve bearing system, the shaft needs to be somewhere between 35 and 55 on the Rockwell C scale. The ability to harden a shaft depends on the material being used and the hardening process (see Table 1).

TABLE 1 Common Shaft Material Characteristics Material

Carbon steels Tensile strength, (lb/in^2 ) Hardenable Characteristics

1018 62,000 Carburize Will corrode 1050 105,000 Heat treat Will corrode 1095 140,000 Heat treat Will corrode 1117 71,000 Carburize Good machinability 4140 148,000 Heat treat Good machinability Stainless steels

303 90,000 No Nonmagnetic-high wear when used with bronze bearings 416 120,000 Yes Free machining 440 260,000 Yes Corrosion resistant

Case Hardening. Case hardening is a process of surface hardening involving a change in the composition of the outer layer of an iron-base alloy followed by appropriate thermal treatment. In order to harden low-carbon steel it's necessary to increase the carbon content of the surface of the steel so that a thin outer case can be hardened by heating the steel to the hardening temperature and then quenching it. The first operation is carburizing to impregnate the outer surface with sufficient carbon; the second operation is heat-treating the carburized parts so as to obtain a hard outer case and at the same time give the core the required physical properties.

The term case hardening is ordinarily used to indicate the complete process of carburizing and hardening.

Some of the most common processes are described here.

Carbonitriding. A case-hardening process which causes simultaneous absorption of carbon and nitrogen by the surface.

Carburizing. A process in which carbon is introduced into a solid iron-base alloy while in contact with a carbonaceous material. Carburizing is frequently followed by quenching to produce a hardened case.

Cyaniding. A process of case hardening an iron-base alloy by heating in a cyanide salt.

Nitriding. A process of case hardening in which an iron-base alloy of special composition is heated in an atmosphere of ammonia or while in contact with nitrogenous material.

3.6 ROTOR ASSEMBLY

The rotor assembly consists of a die-cast rotor and a shaft. Both components may be completely machined and assembled, partially machined and assembled, or a combination of both. The reasons for the various rotor assembly options are economics; size; unit volume; and desired electric motor efficiency, which relates to concentricies and the air gap between the rotor and stator.

3.6.1 Basic Assembly Process

The most efficient and economic assembly method is to assemble a nonmachined die-cast rotor with a completely machined shaft. Probably the most economical assembly method is to mechanically press-fit a shaft onto the rotor. This process can be hand operated or completely automated, with the process determined by unit volume and the variety of rotors and shafts. Less variety and smaller sizes lend themselves more to automation. Some companies have completely automated this assembly process.

Thus, in the basic process flow, illustrated in ill. 17, a completely machined shaft and a nonmachined die-cast rotor are processed by being mechanically press-fit.

This process is used for many very small motors. The resulting rotor assemblies probably won't have the best tolerance concentricities, thus affecting motor efficiencies and noise-but, as mentioned, they will be the least costly.

ill. 17 Rotor assembly process.

3.6.2 Rotor Machining

ID Machining. Cast-aluminum rotors tend to be banana-shaped due to the heat and sometimes due to lack of internal support in the casting process. Also, rotor laminations need to be rotated prior to die-casting in order to eliminate or reduce the lamination material camber, which will cause a banana shape; this is more prevalent in the heating and shrinking process than in the press-fit process. Part of the rotor core bore curve is imparted to the shaft. The rotor core is then turned to obtain the proper air gap. In service, the rotor heats up the aluminum bars, which expand more than the steel core, thus relieving the axial clamp and imparting the curve to the shaft. This can cause unbalance, increase slot-pitch noise, and generate structure-borne noise because of vibration. This effect is greater for long cores.

To solve this banana-shape problem, manufacturers ream, bore, or broach the core ID prior to shaft assembly. This surface then can be used for location when machining the rotor OD if required.

OD Machining-Rotor Only

Turning. Rotor OD machining is usually done on an expanding ID arbor. This allows turning the OD to the average bore diameter. If the rotor bore is machined prior to OD machining and used as a locator, there will be excellent concentricity between the bore and the OD. This possibly might eliminate machining the rotor OD when attached to the shaft, but laminations with the OD punched to size are needed.

Grinding. Rotors that have their ODs cut with a tool will have OD smearing.

This causes lamination shorting at the air gap and will reduce efficiency and cause hot spots. Plunge grinding, with or without a shaft attached, will reduce smearing to a minimum. Some hermetic motor manufacturers use a centerless belt grinder to size and clean up the OD only. Sometimes, depending upon the application, the OD will be used as a locator to machine the ID.

OD Machining-Rotor on Shaft. There are several schools of thought on how to finish-machine the rotor and shaft combination. One method is to allow stock on the bearing journals and then turn the rotor OD and journals in the same setup. In some CNC machines, the journals can be finished to size (not better than 0.0004 in) and finish [20 to 30 root mean square (RMS)] without grinding. This operation can also be completed with a plunge grinder, but the labor content makes it expensive. Completing the bearing journals and rotor OD in one setup is probably the best operation for obtaining consistent air gap.

3.6.3 Electrical Efficiency Improvement Processes

Most motor manufacturers need to have better electrical efficiencies than that pro-vided by the basic assembly process (Sec. 3.6.1), and the machining processes for rotor assembly will affect the required efficiencies. This subsection examines many of the various processes that will improve electric motor efficiencies.

Rotor Machining. Most manufacturers machine the rotor outside diameter and shaft diameters after assembly, but there are other various ways to accomplish this process. The major interest is to achieve better electrical efficiencies. One must have the best concentricity between the shaft bearing diameters and the rotor outside diameter while leaving an equal amount of back-iron thickness. Back-iron thickness is defined as the distance between the rotor OD, which is the lamination, and the aluminum die-cast slot, as shown in ill. 3.18. Also, there is a concern that machining the rotor OD may "smear" the steel laminations and aluminum die-cast materials. This smearing will reduce electric motor efficiencies. In a turning operation, the cutting tools must be maintained in a sharp condition. This rotor OD turning operation can be achieved as a separate part or as a rotor assembly.

ill. 3.18 Rotor back-iron thickness.

Rotor Grinding. Another process method to reduce smearing is to use a center-less abrasive belt grinder to grind the rotor outside diameter (not as a rotor assembly).

The abrasive belt won't become loaded up with steel and aluminum as might occur with a hard-wheel grinder. This centerless grinding process also guarantees that the back-iron thickness will be uniform.

Lamination Punching. Some motor manufacturers punch the lamination to size, thus eliminating any rotor OD turning. However, the lamination dies must be maintained and the process monitored continuously.

Process Options. Several process options are shown in Figs. 3.1 9a through 3.1 9e.

The optimally efficient process for electric motors is probably the one shown in ill.

3.1 9e. Again, with the variety of machining options, a company must evaluate the requirements for electric motor efficiency and economics.

3.6.4 Options for Attaching Rotor to Shaft

ill. 3.19 Options for optimum efficiency electric motor manufacturing process.

ill. 3.20 Rotor and shaft processing option.

There are basically four options in attaching the rotor to the shaft: (1) press-fitting, (2) heating and shrinking the rotor, (3) slip-fitting with adhesive, or (4) welding. The process selected is usually dictated by economics. These processes are discussed here (see ill. 3.20).

Press-Fitting. The most basic and economical attachment process is pressing the rotor onto the shaft. This is usually done in a vertical hydraulic press. The rotor is placed in a holding fixture, and the shaft is placed into the rotor ID. Tolerance control of the rotor ID and the shaft OD must be maintained. Generally, the press fit should be in the range of 0.001 in per inch of shaft diameter minimum.

If the press fit's too tight, the shaft may bend. If the press fit's too loose, the shaft may turn on the rotor in application. Monitoring of the press hydraulic pressure during the press fit will provide a quality assurance check (preventing too tight or too loose a fit).

Usually the shaft rotor diameter will be upset in some manner-knurling, jab blocking, etc.-in order to ensure a press fit.

Heating and Shrinking the Rotor. Another attachment process is heating the rotor by induction or with some type of external heat source and dropping the rotor onto the shaft. The heating process requires energy, and one must be concerned with personnel handling hot parts. It also requires some in-process inventory in the rotor-heating process. This process provides a greater tightness between rotor and shaft than does press-fitting, plus it does not have the same potential for bent shafts as does the pressing operation.

For common shaft rotor tolerances, the rotor should be heated to between 400 and 45 0 F (204 and 23 2 C), but not above 75F (37C), as this temperature will start to affect the aluminum.

Generally, assembly of larger motors, over 5 hp, will use rotor heating because a very large hydraulic press is required for press-fitting.

Section 3.6.5 gives example calculations for determining shrink-fit dimensions.

Slip-Fitting with Adhesive. The rotor ID and shaft OD are sized to allow a slight slip fit of the rotor onto the shaft. It is usually on the order of 0.001 to 0.002 in of clearance. The exact clearance is a function of the adhesive and must be adjusted in accordance with the recommendations of the adhesive supplier. Parts must be clean and free of lubricants before assembly. A drop or two of adhesive is put on the shaft.

It is then slipped into the rotor with a twisting motion. A fixture with a stop is necessary for proper shaft location. After assembly, the adhesive is given time to cure.

Welding. On 5 hp and higher motors, some manufacturers weld bead the final rotor-to-shaft attachment. Others use a key to ensure a locking condition.

Balancing. After the shaft and rotor are assembled, balancing is required. Most balancing operations are done by setting the rotor assembly with the bearing journals on support rollers and rotating the assembly to determine the out-of-balance condition in two planes.

There are two types of balancers, soft- and hard-bearing. Basically the difference is that a soft-bearing machine operates below the suspension's resonant frequency.

Hard-bearing balancers are generally easier to use, safer, and provide a rigid work support.

Most balancing machines will determine the location and amount of weight that needs to be applied. Some motor manufacturers add an epoxy weight to the rotor core. However, a fast drying heat is required in order to speed up the hardening of the epoxy. Others design the rotor end casts with protrusions so that weights (washers) may be added. Very few drill or machine out weight because this can affect electrical efficiencies.

Balancing machines come in either manual- or automatic-load types, usually with computer controls.

3.6.5 Shrink-Fit Calculation Examples

1. Determine the temperature differential delta T, delta F from room temperature).

[delta T (differential expansion)/(basic shaft diameter, in)] / coefficient of thermal expansion where the differential expansion is the total diameter change required. It includes the inference fit plus the slip clearance.

Some common coefficients of thermal expansion are listed in Table 3.2.

TABLE 3.2 Coefficients of Thermal Expansion for Common Materials

Material | Coefficient of thermal expansion, in/(in delta F)

Common steel 0.0000065 Nickel steel 0.0000070 Cast iron 0.0000062 Aluminum 0.0000124

2. Calculate the desired expansion and shrinkage to find temperature change required for 1020 CRS, where the shaft OD is Ø1.2500 and the rotor ID is Ø1.2480.

These diameters give a 0.002-in interference fit. The minimum desired slip fit clearance is 0.003 in, and the differential expansion is 0.005 in.

The temperature change delta T required on these parts to give 0.005-in expansion is calculated as follows.

Delta T = [ 0.005/1.2500] / 0.0000065 = 615.3 F (3.1) A 615.3 F change in temperature is required. Therefore, the total temperature would be 615.3 F plus ambient (7 0 F in this case). One could heat the rotor to 687.3 F (364.1 C). The shaft temperature could be reduced to shrink the shaft in order to reduce the heat needed for expansion of the rotor.

For instance, cool the shaft to  7  F  5  C), and heat the rotor to 540.3 F (282.4  C) to get the required deferential expansion.

3. Another method is to use the maximum change in temperature to determine differential expansion. The total possible change in temperature using dry ice at

 10  F  7  C) to cool the shaft and an oven at 70  F (37  C) to heat the rotor is 80  F (44  C).

 T  80  F (Differential expansion)/(slot OD) / 0.0000065  0.0065 in shrinkage growth (3.2) Differential expansion  T (coefficient of the thermal expansion) (shaft OD) 

(80  F) [0.0000065 in/(in  F)] 1.250 in  0.0065 in 0.0065 in  0.002 in interference  0.0045 in clearance at these temperatures.

For practical purposes, one may use a dry ice temperature of  7  F and an oven temperature of 65 F. This allows for the extremely fast warming of the shaft and cooling of the rotor while assembling.

A minimum of 0.003 in clearance was calculated for all fits. The usual finished interference between the parts ranges from 0.0005 to 0.003 in.

3.7 WOUND STATOR ASSEMBLY PROCESSING

Wound stator assembly processing basically consists of attaching a wound stator core into a housing. However, there are many different assembly processes, depending upon the housing material, the size, and the electrical efficiency requirements.

3.7.1 Steel Pressing

The steel housings are pressed over the wound stator core. Sometimes the mounting base is welded or screwed to the housing before or after this operation. Usually a final attachment is made either by welding beads or by pinning, which requires drilling a hole into the wound stator core.

3.7.2 Cast-Iron Pressing

The cast-iron process, for motors up to about 25 hp, is the same as that used for steel housings, for motors above 25 hp. The housing is heated. For motors above 25 hp, the hydraulic press needed becomes very large, and the process is sometimes not economical.

3.7.3 Heating and Shrinking

Almost all aluminum housings are heated and shrunk onto the wound stator core.

Usually, the housings are pinned to the wound stator core. The base is sometimes welded or screwed to the housing before or after this operation.

3.7.4 Electrical Efficiency Requirements

The mounting of the end frames to the housing is crucial in maintaining the best air gap (concentricies) possible. The end frame bearing housing is usually machined at the same time as the housing attachment diameter (see ill. 3.21).

ill. 3.21 Mounting of end frame to housing.

In order to maintain the best possible concentricies for best air gap control, most manufacturers machine the housing end frame diameter as a wound stator assembly, locating off the bore (see ill. 3.22).

ill. 3.22 Machine housing.

3.8 ARMATURE MANUFACTURING AND ASSEMBLY

Armature manufacturing and assembly require significant hand labor, although the size and unit volumes will dictate the degree of automation. Following is the process and manufacturing flow.

3.8.1 Armature Core Assembly

1. Stack laminations to proper length. Sometimes this is done by weighing the stack.

The outer end laminations are turned so that the burrs are on the inside. Two stacks are made.

2. Place the two stacks in a press. Locate a machined shaft on top of the stack and press into location.

3.8.2 Armature Coil Assembly

1. Insert insulation paper into the armature core lamination slots. This is done either manually or automatically.

2. Insert armature coils (usually rectangular-shaped copper wire) into the armature core lamination slots. This can be done manually or automatically.

3. Twist the ends of the armature coils. This requires a special machine.

4. Press the commutator onto the armature coil.

5. Press or stake the armature coil ends into the commutator.

6. Band the armature coil ends into the commutator.

7. Varnish.

8. Turn the commutator to achieve a very smooth finish. Sometimes a diamond tool is used.

9. Braze the commutator ends.

3.9 ASSEMBLY, TESTING, PAINTING, AND PACKING

The final motor assembly, testing, painting, and packing process is as varied as the other processes, depending upon the unit volume, size, and variety (see ill. 3.23).

3.9.1 Assembly

Most very small motors up to about 1/.4 hp can be assembled automatically. Some are assembled in as little as 5 s by highly automated equipment costing hundreds of thousands of dollars. The success of high-volume automation is the quality of the parts. Part quality that can't be controlled will jam the machine and cause poor utilization.

Changeover from one motor size or configuration to another isn't easily done on a high-volume automated machine, although there have been strides in recent years to provide for quick setup. High-volume automated assembly machines are best run without changeover because they need to be kept running as much as possible in order to justify their cost.

In a line loader, the operator gathers various parts needed to assemble a particular motor (end frames, rotor assembly, wound stator assembly, and miscellaneous parts) and places them on a tray, which moves down a conveyor to the assembly line.

The conveyor to the assembly line is controlled by an assembly operator, so the motor parts may be called for as they are needed.

There are several assembly stations, and each operator takes parts from the tray and completes their portion of the assembly. The operator then places that component on the tray, and it moves to the next station. Sometimes this process is done on a moving conveyor rather than trays.

Some low-volume or significantly sized motors are assembled in one-person cells.

The components are set on pallets and /or in racks for the assembly operator's access.

Usually the operator will assemble the complete motor.

3.9.2 Testing

A variety of electrical and mechanical tests are usually completed on a motor before shipping, some dictated by customer requirements. These tests can be any or all of the following: input voltage regulation, full load, no load, inertial load, equivalent circuit, locked rotor, low-voltage start, torque/speed curve, rotation direction, ac or dc high potential, insulation resistance, surge/impulse, vibration, acoustic noise, and temperature. Most test systems have preprogrammed menus so that the operator does not need to input the data.

After the initial setup, the operator loads and secures the motor into the fixture, makes the proper connections, and starts the test. All testing is automatically sequenced. After the test is completed, the measured results are compared to pre-programmed limits. The data is stored, printed, or transmitted as the user requires.

The operator can observe whether the motor has passed or failed and can take appropriate action. Manual to completely automatic test equipment is available.

Sometimes mechanical tests are performed, such as for rotor assembly end play and tight bearings. Noise tests are also conducted, and most motor manufacturers enclose the entire test area in a sound booth so that any noise can be measured or heard.

ill. 3.23 Assembly and testing. Stator Lam Rotor Cast Rotor Assembly Shell

3.9.3 Painting

Environmental regulations have largely restricted the type of paint that can be used.

Most manufacturers have changed to a water-base or powder paint and have auto-mated the process. Meeting customer color requirements requires motor manufacturers to install equipment that can be quickly changed over.

Some manufacturers paint components before assembly to eliminate masking.

However, the components have to be handled properly in order to minimize marking at assembly.

3.9.4 Packing

Motors under 1/4 hp are usually shrink-wrapped separately. Some have cardboard bases for support. Some are put on a pallet and shrink-wrapped as a container.

Motors over 1/4 hp are usually packed in cardboard boxes or on pallets. Most of the packaging process can be automated as much as possible.

(cont to part 2b)