Ball bearings stabilise optical guidance systems

Super-precision ball bearings

Custom-designed, super-precision ball bearings help to ensure the stability and reliability of optical guidance systems and stabilised camera mounts used in both military and commercial applications.

Optical systems are now employed across many industry sectors, particularly in defence applications but also in commercial uses.

Super-precision ball bearings are critical in ensuring that these systems remain stable and reliable under harsh operating conditions, according to Barney Eley, product engineer at Barden.

In military applications, the latest advanced infrared seeker systems require super-precision ball bearings, which are designed to support the intricate mirror and lens arrangements contained within such optical systems.

Rather than using standard catalogue bearings for these types of applications, optical guidance systems demand the use of bespoke precision bearings, especially if the application is critical in nature or if it requires a bearing solution that offers specific technical advantages such as long life, high reliability, high stiffness and low torque levels.

In order to allow the seekers to look through a broad field of vision, the optical elements are typically mounted in a gimbal system.

These systems usually consist of two axes, elevation and azimuth, which operate independently.

To ensure that the optics are aligned, it is necessary to fix one end of each of these axes; typically, this is performed using a duplex pair of bearings that are clamped to both the shaft and the housing.

At the free end of the shaft, either a similar duplex pair of bearings can be used, clamped only to the shaft, or a spring preload can be applied to a single bearing.

In both cases, clearance to the housing is critical and the bearings must be free to float axially under the effects of thermal expansion.

Typically, optical systems have tight constraints on the size and mass of the installation and, as such, there are restrictions on the amount of power available to the motors driving the optical system.

The internal configuration of the bearings for high stiffness and low torque has conflicting requirements; as a result, the design must be optimised to meet both parameters.

Stiffness, for example, is important when it comes to controlling the precise position of optical elements.

In an angular contact or deep-groove ball bearing, the number of balls, the size of the balls and the curvature of the inner and outer ring and the level of preload can be altered in order to optimise the overall stiffness of the bearings.

Eley said: ’In optical guidance systems used in military weapons systems, Barden typically produces ball bearings that provide high stiffness in order to counteract the high G forces and vibration levels involved.

’The latest infrared seeker systems, for example, perform under high manoeuvring loads and in harsh environments with respect to vibration.

’The bearings must hold the optical components in position at all times, often to an accuracy of a few microns.

’The bearings are, therefore, absolutely critical to the performance of the overall guidance system,’ he added.

In bearing design, lubrication is another key consideration.

For optical guidance applications, wet lubrication is not normally an option when the system utilises lenses and other optical components.

If vapours from the oil or grease condense onto the surface of a mirror, for example, the performance of the guidance system could be compromised.

Most ball bearings for such systems therefore use a dry lubricant so that no oil or grease is present in the system.

According to Eley, for most optical guidance applications, Barden provides bearings in 440C.

This is a martensitic stainless steel that is claimed to provide good corrosion resistance.

To reduce friction and adhesive wear, the bearings can also be designed with retainers that incorporate PTFE as a source of lubrication, particularly for applications in which oscillatory movement of the bearings is required.

Often, the bearings are designed with special ball separators (toroids) positioned over every other ball in the bearing and manufactured from a self-lubricating material.

This ensures that the bearing meets the low torque requirements of the application.

The toroids also enable a high complement of balls to be assembled within the bearing, which helps to increase the load-carrying capability and stiffness of the bearings.

This self-lubricating material is typically virgin PTFE; however, other alternative materials have been developed that comprise a blend of molybdenum disulphide and glass fibre.

For dry lubricated bearing systems, coatings are available for the raceways and balls.

These include MoS2, MoST and silver coatings.

Clearances with mating components must normally be kept to a minimum, as any percussive loads generated by vibration will affect the optical system’s performance.

To attain these, Barden often supplies kits of parts to the customer, pre-matched to controlled clearances in order to simplify and minimise assembly time and costs.

The company manufactures bearings to ABEC7 precision standards, providing run-outs in the micron range.

Super-precision, custom-engineered ball bearings are also required in commercial optical guidance applications, including stabilised camera or video systems and associated mechanical gimbals and gyros.

Here, the bearings themselves may not be directly linked to the camera’s optical system but are nonetheless still critical in ensuring that the camera system is sufficiently stable during filming.

Vehicle- or helicopter-mounted cameras and digital video platforms require high-performance stabilisation systems in order to compensate for any erratic movements of the vehicle, providing a stable, steady platform for filming to take place.

The police, armed forces and rescue workers often require these types of systems during their operations.

Barden provides bearing solutions for a range of mechanical gimbal systems and gyros for such stabilised camera platforms.

Here, a gyro bearing spin axis system may utilise a cartridge bearing unit, which incorporates a bearing with two rows of balls, a single outer ring and two inner rings.

The advantage of this design is that the customer can mount the bearings quickly into his or her own assembly.

Normally, the customer simply mounts on the outer ring by clamping the two inner rings together.

’By manufacturing a bearing to have a precise gap between the two inner rings, a specific preload can be consistently attained when the bearing is mounted in the gyro,’ said Eley.

These gyroscopes will typically be mounted in a gimbal arrangement similar to the optical systems discussed above.

In these situations, it is common for the bearings to be lubricated using the Barden PA cage.

These cages are stainless-steel ribbon cages that are coated on the inside surfaces with PTFE to provide low friction under small oscillatory movements.

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