Gear blanks 1 General

The complete design of a gear requires the design of the gear blank in addition to the design of the gear teeth. An acceptable blank design will meet the following requirements:

-- To provide the necessary size for those gear tooth features which originate in the gear blank; -- To support and position the gear teeth so that they will function as intended;

-- To permit economical manufacture with the desired accuracy.

8.4.2 Design for blank size

8.4.2.1 Outside diameter (or inside diameter on internal gears)

This diameter is usually determined as part of the general gear design process described in clause 6. It may be slightly different in the blank when the gear teeth are to be manufactured by a process in which the finished outside diameter is formed by the cutting tool (called a topping tool). The rough blank outside diameter should then be made greater (or

the blank inside diameter should then be made smaller) by a suitable machining allowance.

8.4.2.2 Face width

The minimum gear face width is determined as part of the general gear design process described in clause 6. The blank face width may be made greater for various reasons, such as:

-- To insure adequate overlap of the face widths of the two meshing gears when axial positioning is not sufficiently well controlled; -- To allow for reduction of the effective face width by rounding or chamfering of the ends of the teeth;

-- To provide adequate face contact ratio.

8.4.2.3 Other blank dimensions

Other blank dimensions, such as the bore diameter, journal diameter or mounting ring diameter, are not part of the gear tooth design. They are usually determined as part of the design of the complete assembly. However, such dimensions, both in size and in accuracy, may influence the operation of the gears and therefore, require the attention of the gear designer.

8.4.3 Design for gear function 8.4.3.1 Support of gear teeth

The shape and proportions of the gear blank should be adequate to support the gear teeth against the transmitted tooth loads. The blank should also be rigid enough to support any other loads tending to deform the blank and its gear teeth. Weight, assembly and manufacturing considerations may require the body of the blank be reduced from the full face width. In such cases, adequate depth of the rim and thickness of the web or cross section of the spokes should be maintained, or ribs should be added, to prevent deflections which could harm the gear tooth action or tooth load distribution. In helical gears, the tooth forces contain axial components that may require additional rigidity of the blank. Similar concerns apply to the features of the gear blank that are attached to the shaft or other supporting member. Where a gear hub is used for this purpose, it should be of sufficient thickness and length. It should be axially constrained to support the radial and axial tooth forces. Whatever the

means used to keep the blank locked to the shaft, such as a key, pin, setscrew, clamp or interference fit, the resulting joint must be strong enough to withstand the applied torque without loosening. Clearance between the gear blank bore and the shaft should be minimized. The same applies to the fit between the key and its keyway, the pin and its hole, or any joining device and its restraining surface. Extra strength and an interference fit may be needed in applications with reversing or abruptly changing loads.

8.4.3.2 Position of gear teeth

The blank design should ensure the gear teeth are positioned concentric and parallel to the axis of rotation of the assembled gear, at least to an accuracy consistent with the accuracy of the gear teeth. The blank features that are to provide this accurate positioning are referred to as the mounting surfaces. Attachment to the shaft or other support- ing members must also be considered. The mating surfaces, if not in an interference fit or tightly clamped, should have minimum clearance. The method of fastening should be such as to preserve concentricity and parallelism. A loose fitting bore fastened to the shaft with a set screw, for example, may result in the loss of the concentricity maintained separately in the gear and shaft.

8.4.4 Design for manufacturing 8.4.4.1 Process requirements

The shape and proportions of the gear blank must be compatible with the manufacturing process. Where the gear teeth are to be formed by a moving tool, the blank design must permit access to the tool, both at the beginning and the end of its forming stroke. Where the gear teeth are to be formed in a mold or die, the blank design must meet ejection and other requirements of the forming process.

8.4.4.2 Location requirements

When the blank manufacture is to be followed by a separate gear tooth machining or finishing opera- tion, the blank design should contain provision for its location during these operations. Ideally, the surfaces used for location during machining will be the same as the mounting surfaces that are to be used to locate the gear in assembly. For example, the bore might serve both these purposes on an external gear and the outside ring diameter serve

these purposes on an internal gear. Often two surfaces are needed, both in manufacture and assembly, one to center the gear blank and the other to keep it parallel to the rotation axis. Sometimes the one or two assembly mounting surfaces are not suitable for location during manufacture. This may be because the mounting surfaces are not accessible to simple tooling or because they will not permit the simultaneous machining of several blanks for greater economy. In this case, some features should be added to the blank design to provide these manufacturing loca- tion surfaces. These surfaces should be accurately located relative to the assembly mounting surfaces. For example, center holes may be added to the ends of the gear blank journals and used for location during the gear manufacturing. The journal diame- ters are made concentric to the center holes. It is usually advantageous to consult the gear manufac- turer during the blank design process, especially if the gear blanks are to be supplied in finished form to the gear manufacturer.

8.4.4.3 Clamping requirements

Clamping of the gear blank during machining of the gear teeth is another consideration in the blank design. The blank design should permit secure clamping without any reduction in the accuracy of the machined gear and without any permanent deformation of the gear tooth surfaces. Some- times, the locating surface can also serve as the clamping surface as, for example, when the gear is clamped by gripping its bore with an expanding arbor. At other times, two separate surfaces are required, as when the gear is centered by an arbor through its bore but is clamped across its face against a shoulder on the fixture. In such a case, if the face of the gear is not square to its bore, the blank (or the arbor) may be distorted during clamping. The machined teeth in the unclamped gear will no longer be accurately positioned relative to the bore.

Clamping considerations may require that some part of the blank be extended beyond its specified shape, with the excess to be removed after the teeth are completed. When the gear manufacturer is also to make the blank, such features can be readily introduced where required. When the blank manufacturer and the gear manufacturer are to be

separated, such features should be included in the blank design.

8.4.5 Blank tolerances

Blank dimensions play an important role in gear performance, either directly or through their influ- ence on assembly or manufacturing accuracy. The tolerance selected for each dimension should be appropriate to the role played by the dimension and to the overall size, pitch and quality level of the gear. Suggested procedures for selecting these toler- ances are described below. Specific applications may require tighter or looser tolerances depending on the relative importance given to the gear performance versus gear blank manufacturing cost.

8.4.5.1 Outside diameter (or inside diameter on internal gears)

The tolerance on this dimension has a direct effect on the depth of engagement and, thereby, on the contact ratio of the mating gears. The direction of the tolerance should be to reduce material, i.e., negative on an external gear and positive on an internal gear. This is to help prevent interference of the tooth tip with the fillet of the mating gear and to help ensure adequate root clearance. The suggested tolerances are:

Outside diameter tolerance =

...(62) + 0.0

−



nd



Inside diameter tolerance =

...(63) +



nd



− 0.0 where

Pnd is diametral pitch on spur gears and normal

diametral pitch on helical gears;

TCT is total composite tolerance.

The above discussion and suggested tolerances apply only to the diameter of the finished gear. If the blank diameter is modified in the cutting (topping) operation, the tolerance on the original diameter should be derived from other requirements, such as the need to control the amount of material to be removed.

8.4.5.2 Concentricity of outside diameter (or inside diameter on internal gears)

The tolerance on the concentricity of the finished diameter relative to the bore or other cylindrical mounting surface is similar to the diameter toler- ance in one respect. By allowing variations in the radius, it permits greater reduction in depth of engagement and, hence, in contact ratio. This additive effect should be kept relatively small. The suggested concentricity tolerance is:

Tolerance = 0.7(TCT) + 0.0002 ...(64) This tolerance may be increased if there is an equivalent reduction in the diameter tolerance. The tolerance may also be increased if the gear design has been checked for the resulting minimum contact ratio. See clause 6.

One special situation arises when the gear cutting set up will center the blank by its outside (or inside) diameter instead of by its bore or other mounting surface. If the blank is made by the same shop that cuts the gear, this shop will set its own tighter blank concentricity tolerance. If not, the gear manufacturer should be consulted before blanks are made.

8.4.5.3 Other blank dimensions

There are other blank dimensions whose tolerances can influence the performance of the finished gear. These include the diameters of the bore, of the journals, and of other cylindrical surfaces used for mounting or manufacture. They also include the concentricities between pairs of these surfaces and the squareness between these surfaces and the lateral mounting or manufacture set--up surfaces. For each of these, there are various assembly and manufacturing conditions which determine the effect of the tolerance and the selection of its value.

8.4.5.3.1 Clearance fit assembly

When a cylindrical surface might be assembled off--center with respect to a shaft in a clearance fit, a larger diameter tolerance permits a greater clear- ance and, therefore, a greater off--center condition. The clearance fit of a bore on a shaft is an example of this.

8.4.5.3.2 Rotating fit assembly

When a cylindrical surface on a blank is to rotate about a stationary member, the added clearance permitted by the diameter tolerance does not affect concentricity in the gear meshing action. It may, however, increase center distance and reduce tooth engagement. A bore rotating on a stationary shaft or a journal rotating in sleeve bearings are examples of this assembly condition.

8.4.5.3.3 Interference fit assembly

When a cylindrical surface on a blank is to be assembled to the supporting member with an interference fit, there is usually no change in concentricity or center distance. The assembly of a shaft into the inner race of a ball bearing and the assembly of a thin rim gear on a shaft--mounted wheel are examples of this assembly condition. The diameter tolerance is usually determined by the requirements of the interference fit. In the case of the thin rim gear, however, large variation in the amount of interference may cause a corresponding variation in the size of the stretched gear and in the resulting depth of engagement. This may present a problem in very fine--pitch gears and the diameter tolerance may need to be reduced further.

8.4.5.3.4 Gear cutting set--up

When the blank might be centered differently in the gear cutting set--up than in the final assembly, a loss in concentricity may be introduced. This may occur in two ways:

-- Different surfaces of the blank are used for the two location purposes. For example, center holes in the ends of the journals are used in the gear cutting set--up while the journal diameters are used in the final assembly;

-- One surface of the blank is used for both lo- cation purposes, but its diameter tolerance per- mits a wide range of size. An off--center condition may arise when the size of the location device in the gear cutting set--up cannot adapt to the blank size variation. In the case of the bore, if the arbor used to center the blank is made to fit the mini- mum bore diameter, it will not necessarily center the blank with maximum bore diameter. If many blanks are to be located in one set--up on the same arbor, variations in the bore diameters may result in off--center gears.

The tolerances needed to control these manufac- turing concentricity variations may need to be tighter than those needed to meet assembly requirements. These tolerances are best selected by the gear cutting shop, either by making its own blanks or by advising the blank designer.

8.4.5.3.5 Lateral surface assembly

When the gear is to be assembled using a lateral surface to keep the gear teeth parallel to the shaft axis, any non--squareness between the lateral surface and the gear teeth will affect gear perfor- mance. Such variations usually introduce an eccentric condition into the action of the meshing gears and some degree of tooth misalignment. If the same lateral surface used to square the gear in the assembly is also used to position the gear during manufacture, the blank will not add any non--squareness. If different surfaces are used, it is necessary to introduce a tolerance relating the two surfaces. This tolerance may be a lateral runout specification, a parallelism specification or both. When serving purely as in--process tolerances, these are best set by the gear manufacturer. When these tolerances are required in order to meet special assembly conditions, their values should be selected after considering lateral surface diameter, gear face width and the amount of eccentricity and misalignment that can be tolerated.

8.4.5.4 Geometric form

It may be necessary to establish tolerances on various features of geometric form, such as round- ness and flatness, in order to meet especially stringent assembly conditions. Such conditions may arise when the tightness of interference fits and clamped surfaces must be carefully controlled. They may also arise when the gear blanks are sufficiently flexible that the tight fit or clamping of poorly formed surfaces may distort the gear teeth and reduce their accuracy after assembly. In this last case, variations in form are less critical if the gear blank will receive the same kind of support during manufacture of the teeth as in the final assembly. The decision to use a form tolerance and the selection of its value should be based on an evaluation of these special assembly conditions.

8.4.5.4.1 Bore

Form tolerances may be required on bores or other inside diameters used for assembly for the condi- tions of out--of--roundness, taper, bell mouth, and barrel shape.

8.4.5.4.2 Journals

Form tolerances may be required on journals, or other outside diameters used for assembly, for the conditions of out--of--roundness and taper.

8.4.5.4.3 Lateral surfaces

Lateral surfaces used for assembly may require a tolerance on flatness. Usually, convexity is not permitted and the tolerance is expressed as a limit on concavity.

8.4.6 Drawings

On the gear drawing, the dimensions and toler- ances of the gear blank are customarily applied to the views in accordance with standard drafting practice. Only the outside diameter and its tolerance are usually transferred to the format table. A separate drawing for the gear blank may be made if the blank is to be obtained from a source other than the gear shop. Such a separate drawing should show the larger outside diameter (or smaller inside diameter) if it is to be cut during gear cutting. It should also show any extra stock remaining for clamping during cutting and to be removed later.

9 Gear tooth tolerances

Clause 5.7 of this information sheet discusses application considerations of gear system design. Tolerancing of gear teeth cannot be done correctly without considering the application of the gears. It is obvious that the closer a gear is to perfect, the better it will perform in any application. The difficulty is in determining the character and amount of variation from perfection that a given application will tolerate. For example, if noise of a gearbox is of prime importance, it can be shown that frequency of excitation is an important factor. Depending upon where in the geartrain a gear operates, gears are toleranced differently. The tooth profile variation may be more important at the low speed end of the geartrain, whereas, the accumulated pitch variation

is more important at the high speed end of the geartrain.

It is difficult to generalize the process by which gears are correctly toleranced. The following paragraphs attempt to offer insight into the fundamental concepts of tolerancing. Each gear must be carefully analyzed for functionality in order to tolerance it correctly.