Introduction to Gear Design

2 What Kind of Gears Should I Use?

Successful gear systems often depend as much on selecting the right gear for the job as on the proper design of the individual parts. Gears can be made in a wide variety of forms, each with its own strengths and weaknesses. In some applications different gear types can be used with equal success. There are other cases where a specific type of gear has become the “standard” due to its unique characteristics. Table 2.1 shows the most common kinds of gears, organized by shaft orientation, showing their relative characteristics compared to other types of the same shaft orientation. Additional comments on each are made in the following paragraphs.

2.1 Parallel-Shaft Gears

Spur gears are by far the most common type of parallel-shaft gear. They are simple to design, highly efficient, and relatively forgiving of mounting errors. Spur gears can handle high horsepower and shock loads but are not the most compact way to transmit power due to the relatively low contact ratio that can be obtained. Contact ratio is a measure of smoothness of operation and is related to the number of teeth in contact (and sharing the load) at any one time. Well-designed spur gears should never have a contact ratio of less than 1.2, but it is hard to get a contact ratio much over 1.8 without employing a non-standard “high contact ratio” tooth form, for which special tooling is required. Spur gears do not generate thrust forces (loads in the direction of the shaft axis), which allows for much simpler housing and bearing arrangements.

Helical gears are often thought of as “twisted” spur gears because the teeth run at an angle to the shaft axis. This “helix angle” is produced by setting the cutting tool at an angle to the workpiece and using a differential to vary the relative speed of rotation between the tool and the workpiece. The helix angle raises the contact ratio by bringing more teeth into contact across the face of the gear. This “face” contact ratio is added to the “profile” contact ratio of the spur gear to give a “total” contact ratio that can be tailored to meet higher load requirements and operating speeds. There is a thrust load created by the helix angle that complicates bearing selections, however. Analysis of bearing loads can be complex. Consult your bearing manufacturer or one of the reference books for suggested analysis methods.

Double-helical gears have two “opposite-hand” helical gears on a single shaft. This theoretically creates equal and opposite thrust forces that cancel each other, giving the advantages of helical gears without the bearing-load problems. In practice, however, it can be difficult to insure that each “helix” carries an equal load. External thrust loads caused by coupling miss-alignments or imbalance can interfere with the ability of the gears to “float” axially and find their equilibrium point. This causes one side of the gear to carry more load and wear out sooner. The design of mounting and bearing arrangements for double-helical gears turns out to be just as difficult as for helical gears. These gears can handle very high loads and operating speeds, which accounts for their popularity in pump drives and marine propulsion units. A great deal of research has been published on the system dynamics of these drives but much of it may be difficult for the non-expert to use.

Internal gears can be made in spur or helical forms. Contact ratios are slightly higher than for external gears of the same proportions, but load-carrying capacity suffers from face-width limitations and an inability to mount adequate bearing on the pinion. The internal gear is also very awkward to mount, which can make the drive difficult to package.

Planetary gears use multiple gear meshes inside an internal gear. These meshes have the effect of canceling the “separating” loads (forces tending to push the gears apart), which reduces the bearing loads. As power capacity is calculated on a “per mesh” basis the planetary-gear design allows for very high loads in a compact space. The “down side” of all this is that lubrication requirements and thermal losses can put limits on the allowable operating speeds unless external cooling and lube systems are employed (which reduces the “compactness” advantage). In addition, a high degree of precision is required in part manufacture to insure that the load is shared equally. There are some specific mathematical relationships that must be maintained in the design of planetary gearsets, which can restrict the ability to obtain exact ratios. The best approach for the novice designer is to read everything mentioned about planetaries in the reference books and to look carefully at existing installations.

2.2 Intersecting-Shaft Gears

Bevel gears are the most popular means of connecting intersecting shafts. Straight-bevel gears (including Coniflex) have much in common with spur gears, while spiral-bevel gears (including Zerols) are similar to helical gears in operating characteristics. All bevel gears are extremely sensitive to mounting accuracy, and require careful analysis of bearing loads. The Gleason Works has a lot of information on the design of bevel gears and mountings. The published limitations on proportions and numbers of teeth should be strictly observed. Doing otherwise can lead to unsolvable manufacturing and field problems.

Face gears are not commonly used in power-transmission designs due to their low power capacity and lack of standardized calculation procedures. Face-gear design information is available in some of the reference books listed at the end of this manual. These gears can be useful in some timing and indexing applications. Consult an experienced manufacturer before designing any “new” face gears, as tooling considerations are critical.

2.3 Non-Intersecting-Shaft Gears

Worm gears were originally designed as “jacks” for raising and lowering weights. They are uniquely suited for static-load applications because of their tendency to “self lock” under certain conditions. “Self locking” occurs when the worm can turn the gear but the gear cannot turn the worm — the load cannot cause the drive to backup. This phenomenon does not occur in all wormgear sets and should not be counted upon to take the place of a brake for safety-related applications. Wormgears can also provide the highest possible reduction ratio in a single “pass” and are just about the only type of gear where the gear-diameter-to-pinion-diameter ratio does not correspond to the reduction ratio. This allows modern power-transmission wormgear boxes to be very compact in comparison to other gearboxes of similar reduction. The large amount of sliding action in worm meshes can result in low efficiency and power limitations due to thermal losses. The meshing action is very smooth, making these gears ideal for indexing applications. Calculation procedures are available through AGMA and in most reference books.

Crossed-helical gears can be thought of as a simple form of “non-enveloping” wormgear. Load capacity is severely limited because of the small contact area between the gear and the pinion. These gears are inexpensive to make and are very forgiving of mounting errors, however, which makes them popular for low-power “takeoffs” or timing purposes (like packaging machines). Calculation procedures are not standardized through AGMA but can be found in some reference books.

Hypoid gears are a modified form of spiral-bevel gear. All comments made about bevel gears apply here as well. Rear-wheel-drive auto and truck axles are the most popular use of Hypoid gears.

“Face worm” gears have been sold under the trade names “Helicon” and “Spiriod”. The original design patents have now expired and there is nothing to prevent a “second source” from developing similar gears. The proprietary nature at these gears has tended to make them more expensive than worms or Hypoids, and less well understood. The design and rating methods that have been published for these gears have not been independently tested or sanctioned by AGMA. It appears these gears share some characteristics with worms and Hypoids, but may have other weaknesses or strengths.

Face gears can also be designed for non-intersecting shafts. The comments previously made about face gears apply here as well.