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Product category: Gears, brakes, couplings and engines
News Release from: Danaher Motion | Subject: Redimount
Edited by the Engineeringtalk Editorial Team on 17 June 2005

The essentials of servo gearhead
selection

Gearheads are used throughout a vast range of engineering applications, but to select the correct gearhead for a specific purpose a broad range of requirements must be considered.

Gearheads are used throughout a vast range of engineering applications, but to select the correct gearhead for a specific purpose a broad range of requirements must be considered The most obvious are the physical dimensions and torque capacity, with manufacturers placing great emphasis on the performance figures such as backlash

However, there are a great deal more hidden or misunderstood requirements that must be fulfilled.

This article will illustrate the important decisions that the design engineer is required to make in the context of the underlying technology and design of the gearheads themselves, focusing on servomotor applications.

Finally, the simple cost-saving solution of purchasing matched or integrated gearheads with a servomotor is considered.

The single most important reason for using a gearhead in a servo system is to increase torque.

Large electric motors are expensive, so it is cheaper to fit a gearbox to a fast rotating motor to produce greater torque than it is to increase the size of the motor used.

The aim is to produce more torque - economically - than is available from a servomotor on its own Secondly, gearheads can also improve the dynamic response of a servo system if the inertia between the load and motor is matched correctly.

The proper inertial match will also minimise torque and power dissipation in the servo system.

The first performance criterion considered is usually the backlash of the gearhead, this is the amount by which the width of a gear tooth exceeds the thickness of the engaging tooth, measured as the angle at the pitch circle of the gear.

For many mechanical applications, such as a car gearbox backlash is of little concern and is in fact necessary to allow space for lubrication of the gear teeth.

However, when backlash occurs in a servo system it will lead to positioning and repeatability errors unless compensated for, particularly if direction is reversed.

In a manufacturing process, or a machining application such inaccuracies are unacceptable; the spacing of the teeth is a balancing act between maximum contact to ensure good power transfer and space for lubricant.

Backlash is measured at the output shaft, with the input shaft fixed under minimal torque.

The backlash can be viewed as the "air" in the system, the space between gear teeth accounting for a lull between the input being rotated and the output beginning to move.

In order to gain a practical measure of the total motion that is lost in a servo system, the concept of lost motion has arisen, that takes into account the lost motion at every point in the system.

In addition to backlash, this lost motion arises from the physical properties of components, how they are constructed and how well they are put together.

For example motion can be lost because of bending shafts and bearings under compression.

It is important to note that if a greater torque is applied to rotate the output, lost motion is increased.

The design engineer would be disappointed when selecting a "low backlash" gearhead to find that backlash is not equivalent to the lost motion experienced, the figure quoted being for an unloaded gearhead.

Lost motion is the principal cause of positional uncertainty in a motion system and can be easily overlooked, resulting in positioning errors in a machine or process.

Lost motion leads to an uncertainty in displacement that is typically nondeterministic in servo applications.

This is because the lost motion is proportional to the force applied, which can vary dramatically in a highly tuned servo system.

The greater the torsional stiffness, the easier it is to control a system's accuracy.

Unlike backlash, lost motion is measured when a torque is applied.

Whereas backlash is a measure of "air losses", lost motion is the concept of all lost motion in the system - including backlash.

Because lost motion varies with torque, torsional rigidity is a useful specification when selecting a gearbox for an application.

Torsional rigidity is defined as the torque applied divided by the resulting angular displacement, measured in Newton-metres per minute of arc.

The gearhead stiffness will fundamentally limit the accuracy of the system's dynamic response, no matter how well the drive and servomotor are specified.

In addition to creating positioning errors, the settling time of the system is also lengthened.

Gearhead manufacturers employ a number of methods to reduce backlash, the first is to preload the gearbox - the principle used in cyclic gearboxes.

A preloaded gearbox reduces backlash to a minimum, but the preloading makes them very inefficient because of the frictional losses introduced.

The torque ripple (input-torque variation torque variation that results when gearheads at low speeds) is also increased so they do not operate smoothly.

Another method for reducing backlash is to improve the mesh alignment by modifying the profile of the teeth.

One technique is crowning, by crowning the gears teeth align better so they mesh more effectively.

This also distributes the load better reducing stress that can cause pitting.

Because the teeth mesh better they also produce less noise, which is becoming increasingly important in modern day manufacturing.

In the most basic gearboxes, spur gears are used.

These have the advantage of being cheap to manufacture and are therefore cheaper parts to purchase.

The disadvantage is that they typically have a contact ratio (defined as the number of teeth in mesh at any given time) of 1.5.

By cutting the gears helically, the contact ratio can be increased to a typical ratio of 3.3 which is more than double that of a spur gear.

The advantages are significant, increasing the effective torque capacity for a given frame size and making operation smoother.

Smoother operation also enables the gearbox to operate at higher speed.

Using both crowned teeth and helically cut gears reduces wear and the greater meshing increases the torque rating of the gearhead.

A basic spur gearhead consisting of a pinion and a single spur has a shorter service life, requiring regular maintenance and lubrication.

For example using two gears the ratio available is limited, but can be increased by adding more stages.

That not only increases the size of the gearhead, but does not help to increase the torque capacity.

To achieve higher ratios and greater torque capacity a planetary gearhead is used.

The external environment also affects the choice of gearhead, for example any heat developed must be dissipated.

This is particularly important in machines where space is at a premium, a high efficiency gearbox will reduce the dissipation requirement.

In many cases this is academic because the servomotor will produce more heat than the gearbox, but correct inertia matching of the motor and the load will help reduce overall heat dissipation.

In corrosive environments, stainless steel gearheads can be used.

These are more costly than standard gearheads, but are essential to ensure reliable operation and long service life in food, chemical, pharmaceutical processes.

A planetary gearhead uses a more sophisticated gear arrangement, with several spur (typically three to five) gears (called planet gears) rotating about a pinion (called the sun gear).

The planet gears orbit within an internal gear which is normally cut into the inner circumference of the gearbox.

This construction is rigid, increasing the torsional rigidity of the gearhead as a whole.

Because the planet gears come into contact with the inner ring, a single drop of oil can effectively lubricate the whole gearhead.

The planet gears all share the load attached to the output shaft, this load sharing means that a planetary gearhead has a higher load capacity than a spur gearhead for a given size of gearbox.

Because it is possible to have a number of gears within a confined space, very high ratios are possible.

Ratios from 1:1 to 100:1 are typical, with ratios a high as 500:1 possible by employing multiple stages.

Gearheads are manufactured to standard sizes such as NEMA frame sizes to match the available motors.

Gearboxes come in a wide range of ratios and are necessarily modular components so that they can be bolted on to a motor to achieve the desired servo system.

The problem is that motors have different shafts, and different mounting standards that are dependent on the manufacturer.

Having discussed the importance of correct load balancing and reduced backlash, all of these gearbox specifications amount to nothing if the gearbox is not properly mounted on the servomotor.

If incorrectly aligned, the gearbox will be inefficient and subject to considerable wear that reduces the service life and smoothness of operation.

In a manufacturing operation, the gearbox must be quick and simple to mount, minimising wasted maintenance time.

For the machine builder, the gearbox must be properly aligned and compact.

In every case it is essential that the correct parts are available at the time that they are required.

In order to align the gearbox correctly with the servomotor the pinion must be correctly positioned between the planet gears.

Setting the position of this pinion can be a time consuming and difficult task.

These problems have led to the introduction of other parts into the system.

In order to install a gearbox, sleeves, bolts and a housing are required.

In fact, machine builders have been known to produce their own housings to ensure that the servomotor is properly attached to the gearbox.

Some gearbox manufacturers offer mounting kits containing the correct parts.

For example, engineers at Danaher Motion has introduced a new mounting kit called Redimount that enables any motor to be mounted with a Thomson Micron gearbox.

The Redimount system includes a pre-installed pinion, avoiding the need for pinion setting, and a housing machined at their factory specifically for the required motor.

A self-aligning hub ensures concentricity.

This reduces the time and effort involved in mounting a Thomson Micron gearbox considerably, in fact with the correct mounting kit it is possible to fit a new gearbox in less than 4min.

An added benefit of the motor-specific mounting kit is the ability to transfer an existing motor to a new gearhead and vice versa.

It is significantly cheaper to swap a simple mounting kit than it is to replace expensive components like motors and gearheads.

Using a universal mounting system such as Redimount has its benefits, but the alternative to purchase the gearhead with the servomotor.

There are two ways to do this: purchase a prefabricated "geared motor"; or buy a matched gearhead and servomotor from a single supplier.

The first option restricts the options available: what if the gearhead is not the ratio you are looking for?.

Perhaps you need a stainless gearbox for washdown applications?.

If the gearhead and servomotor are available from a single supplier, they can match the gearhead and motor to suit the application.

Because the supplier's engineers have matched the servomotor and gearhead, in-house engineering costs are reduced.

There are cost reductions in sourcing the components from a single supplier stemming from the reduction in orders and administration.

Integrated components represent "value selling" for the manufacturer and buying from a single supplier also result in a price reduction.

For example, Danaher Motion can offer a price reduction on its high quality Thomson Micron gearheads if they are purchased with a high power density AKM servomotor.

When choosing a gearbox for use with a servomotor, the engineer must consider the wider picture.

Once the correct ratio and size are established, more practical issues such as service life, maintenance and lubrication are important.

It may be possible to order the gearbox in a week, but how long will it take to match it to the motor and what parts are required?.

The benefits of planetary gearheads are clear, but at what cost?.

The cost of the gearhead may be greater than that of a spur gearhead, but providing the very high output speeds are not required, the planetary gearhead may be cheaper when amortised over time because of the reduction in maintenance and extended service life.

This is particularly true of the new generation of dynamic gearheads discussed.

Noise, although still a secondary consideration is becoming increasingly important in a manufacturing environment because of noise regulations coming into force.

Modern designs with better meshing are significantly quieter than conventional designs.

The pressure on manufacturing in Europe means that downtime for maintenance must be kept to a bare minimum.

This means that a gearhead or motor must be reliable and robust, have a long service life and be easily replaced.

Planetary gearheads are self-lubricating and low wear, and using a mounting system such a Redimount the gearhead or motor can be replaced quickly and easily to reduce unprofitable downtime.

Single supplier procurement and component integration can significantly reduce administration and engineering costs.

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