Reviewed by: Technical Engineering Department – TGB Group Published: september 2026 Technical review: september 2026

Backlash is one of the most misunderstood parameters in slewing systems. It is often treated as a defect to be eliminated, as if the ideal value were zero. In reality, a controlled amount of backlash is normal, necessary and designed in. In geared slewing systems, zero backlash is generally not a practical operating condition, because thermal expansion, lubrication requirements and manufacturing tolerances all require a controlled clearance to be maintained at the gear mesh.

The useful question is therefore not “how do I remove backlash” but “how much backlash is right for this application, and how do I keep it within that range over the life of the system.” Backlash becomes a problem in two opposite directions: when there is too little, and when there is too much. Between those extremes lies a specified range that is correct for each design.

This article explains what backlash actually is, why it must exist, when it is normal, when it becomes a problem, and how to control it both at the design stage and over the service life of the system.

What backlash is

Backlash is the small clearance between the meshing teeth of two gears, measured as the amount of free movement at one gear before the other begins to move when the direction of rotation reverses. In a slewing system, this clearance exists at the interface between the drive and the ring gear: between a drive pinion and the gear teeth of a slewing ring, or between a worm and the ring gear in a worm-driven slewing drive.

In practical terms, backlash is what allows a gear to reverse direction. When motion reverses, the driving tooth must travel across the clearance before it contacts the next driven tooth. That clearance is backlash, and the brief moment in which the output does not move while the clearance is taken up is the lost motion it produces.

Why backlash must exist

Zero backlash is not a goal; it is a failure mode. A meshing pair needs clearance for several reasons.

Thermal expansion is the first. Gears and housings expand as they heat up in operation. A mesh set with no clearance when cold will tighten as temperature rises, increasing friction and, in the extreme, binding or seizing. Backlash provides the room for this expansion.

Lubrication is the second. The clearance between teeth is where lubricant sits and is carried into the contact. Without it, the film that separates the surfaces cannot be maintained, and metal-to-metal contact accelerates wear.

Manufacturing and assembly tolerances are the third. No gear is perfectly round and no mounting is perfectly positioned. A slewing ring gear has a high point of radial runout, a point of maximum eccentricity. Backlash absorbs these real-world variations so that the mesh does not bind at the tightest point of rotation.

For these reasons, every gear mesh is designed with a specified backlash range. The objective of good engineering is not to eliminate it, but to set it correctly and keep it stable.

The two sources of play in a slewing system

An important distinction, often missed, is that the total play a user feels at the output of a slewing system can come from two different sources.

The first is gear mesh backlash, the clearance between the drive and the ring gear described above. The second is the internal clearance of the slewing ring itself, the small axial and radial freedom between the rolling elements and the raceways. Both contribute to the total lost motion measured at the load, and both can increase with wear.

This matters when diagnosing a precision problem. If a system has developed excess play, the cause may be at the gear mesh, inside the bearing, or both. Treating only the gear backlash when the clearance has actually developed inside the bearing, or the reverse, leaves part of the problem in place. Controlling play in a precision application means considering both sources together.

When backlash is normal

Backlash is normal, and not a cause for concern, when three conditions are met. It sits within the range specified by the manufacturer for the gear and application. It was set correctly during assembly, at the high point of the ring gear’s radial runout, so the mesh does not tighten as the ring rotates. And it remains stable over time, rather than increasing as the system operates.

A slewing system that meets these conditions has the backlash it was designed to have. In most industrial rotation, where the system moves loads without demanding fine positional accuracy, this designed-in backlash has no negative effect on performance at all.

When backlash becomes a problem

Backlash turns from a normal feature into a problem in specific situations.

In precision positioning applications. Where the system must position a load accurately and repeatably, backlash becomes lost motion that degrades accuracy. Solar trackers, robotic systems and positioning equipment are the clearest cases: every reversal of direction loses the position by the amount of the backlash until the clearance is taken up. In these applications, backlash that would be irrelevant in a crane becomes a direct source of positioning error, and our article on specifying slewing drives for solar trackers looks at this in the context of energy capture.

Under reversing or dynamic loads. When loads reverse frequently or arrive as shocks, the teeth repeatedly cross the backlash gap and strike the opposite flank. Excess backlash turns each reversal into an impact, producing noise, vibration, accelerated tooth wear and, over time, damage. The larger the backlash, the harder the impact.

When it increases over time. A gradual increase in backlash is a wear symptom. As teeth and raceways wear, the clearance grows, accuracy falls and impact loads rise, which accelerates further wear. Increasing backlash is therefore both a problem in itself and a signal that the system is wearing, a point covered in our article on recognising wear in slewing systems.

When there is too little of it. The opposite extreme is equally a problem. Backlash set too tight, or eliminated to chase precision, causes the mesh to bind under thermal expansion, raises friction and heat, and accelerates wear. In a worm-driven slewing drive in particular, insufficient clearance can sharply increase the torque required to turn the system.

How to control backlash

Controlling backlash is partly a design decision and partly a maintenance discipline.

Set it correctly from the start. The single most important control is correct setting during assembly: locating the high point of the ring gear’s radial runout and adjusting the centre distance between the drive and the ring gear so that the backlash falls within the specified range at that point. This is covered in our article on integrating the structure, slewing ring and drive.

Select a drive designed for the required precision. Where the application demands accuracy, the drive should be chosen for low backlash rather than adjusted to compensate for an unsuitable one. TGB Group’s BE series, for example, can be specified for applications where controlled backlash, accuracy and environmental protection are relevant, with full enclosure and lip seals providing IP65 protection.

Use self-locking where holding position matters. Some worm-driven slewing drives can be self-locking, depending on the gear geometry and operating conditions. In those cases, the output cannot back-drive the worm, helping the system hold position without relying only on an external brake. It is important to be precise here: self-locking prevents the system from being back-driven, but it does not by itself remove the mesh backlash. A self-locking drive still has a specified backlash that must be set and maintained.

Consider preloaded or anti-backlash arrangements for high precision. Where positioning accuracy is critical, anti-backlash methods exist at the system level, such as preloading two drive elements against each other so that the clearance is taken up. These add cost and complexity and are justified only when the application genuinely requires them, which is why the level of precision should be defined before selecting the solution.

Address bearing clearance, not only gear backlash. Because play has two sources, controlling it can also involve the slewing ring itself. TGB offers slewing rings with a double raceway among its configuration options, which can be specified where the application requires greater rigidity and increased load capacity.

Maintain it over the service life. Backlash should be kept within range over time, not only at commissioning. Maintaining correct bolt preload prevents micro-movement that adds to play, and monitoring backlash as part of inspection allows a developing increase to be caught before it degrades accuracy or causes impact damage.

How backlash is measured

Measuring gear mesh backlash in a slewing system is straightforward and requires no specialised equipment beyond a dial indicator and a fixed reference point. The principle is to capture the angular lost motion at the output while the drive is stationary.

The procedure is as follows. With the system stationary and the drive locked or held, a dial indicator is positioned against a rigid point on the rotating structure, as close to the ring gear diameter as possible to maximise the resolution of the reading. A zero reference is established with the structure pushed firmly in one direction, taking up all the clearance. The structure is then moved in the opposite direction until contact is made on the other side of the backlash gap. The total travel recorded on the dial indicator is the backlash, measured as linear displacement at that radius.

If the result needs to be expressed as an angular value, the linear measurement is divided by the radius at which it was taken. The result is then compared against the backlash range specified by the manufacturer for the gear module and diameter in question.

Two points matter during the measurement. It should be taken at multiple angular positions around the ring, because the high point of radial runout makes the mesh tighter at one position than at others. And it should be taken with the bolted joint at full preload, because a loose mounting allows micro-movement that adds to the reading and gives a false impression of excess backlash. Confirming bolt torque before measuring eliminates that variable.

Symptom reference: what backlash problems look like

 

Quick reference: backlash by application

What backlash problems usually look like in the field

In many field cases, backlash problems are first reported as positioning drift, repeated impact noise during reversal, irregular tooth wear or a delay between motor command and visible movement of the structure. These symptoms do not always mean the drive is undersized. They often indicate that backlash has been set incorrectly, has increased through wear, or is being amplified by internal bearing clearance or structural flexibility.

This is why diagnosing a backlash problem means checking both interfaces: the gear mesh and the bearing clearance. It also means checking bolt preload, because a loose mounting allows micro-movement that adds to total play in a way that is easy to mistake for gear backlash. The correct sequence is to confirm the source before deciding the remedy.

Common mistakes with backlash

A number of recurring mistakes appear in how backlash is handled:

• Treating backlash as a defect and trying to eliminate it, causing binding and overheating.
• Setting backlash away from the high point of runout, so the mesh binds once per revolution.
• Assuming all play comes from the gear mesh, when part of it is internal bearing clearance.
• Specifying a drive and then over-tightening the mesh to obtain precision it was not designed for.
• Confusing self-locking with zero backlash; a self-locking drive still has mesh clearance.
• Failing to monitor backlash over time, so a developing increase is only noticed once accuracy is lost.

Where backlash matters most

Backlash is a design parameter in every geared slewing system, but its importance rises sharply in applications that combine reversing motion, dynamic loads or precision requirements. Solar tracking and positioning systems are sensitive to it because accuracy is the function of the system. Equipment with frequent direction changes or shock loading is sensitive because excess backlash becomes repeated impact. In simple, unidirectional or low-precision rotation, designed-in backlash is rarely an issue at all. Matching the attention given to backlash to the demands of the application is itself part of good selection.

How TGB Group supports backlash control

With more than 30 years of experience in motion solutions, TGB Group’s in-house engineering department helps customers define the right backlash for their application and select the configuration that delivers it. This ranges from standard slewing drives for general rotation, to BE series configurations for applications where controlled backlash and accuracy matter, to self-locking worm drives for position holding and double-raceway slewing rings where reduced play and higher rigidity are required.

Because the correct backlash value, the setting procedure and the maintenance criteria depend on the specific product, TGB provides these parameters in the technical documentation for the selected slewing ring and drive, and supports the engineering team in defining and maintaining backlash for the real operating conditions of the application.

If you need to define how much backlash your application can tolerate, or to select a drive for precision positioning, TGB Group’s technical team can help. Contact our technical team to define the right solution for your project.