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Novel auto-focus system relies on custom miniature linear bearings

08 March 2015

Fluke discovered that a custom low friction, self-aligning miniature linear ball bushing bearing could not only improve the accuracy of a new thermal imaging camera guide under development, but could also save money into the bargain.

Fluke’s Ti400 camera range features an innovative auto-focus system called ‘LaserSharp’ that achieves the consistently in-focus images that are essential for accurate infrared inspection. The camera emits a laser beam, which reflects off the target, measuring the time of flight and thus the distance from camera to target. This information is then mapped to the focus mechanism that positions the movable lens to the correct distance from the sensor rapidly and precisely.

The moveable lens is located inside the camera so that the outer lens can be fixed. It is designed to be supported by - and travel on - a miniature linear ball bushing bearing. During development it became clear that plain linear bearings with their inherently high friction would not meet the required specification, as Fluke design engineer, Brian Gattman explains:

“For use in a compact, light-weight, hand-held instrument we needed an extremely small bearing, just 3mm diameter, but it also had to be very low friction to minimise the power requirement for the focus system which is a significant factor in terms of the instrument’s battery life. We found that Thomson off-the-shelf 1/8in linear ball bushing bearings performed well, but the 3mm bearing size we needed was not available in their standard range. 

“After some discussion, Thomson quickly tooled up to produce a 3mm bearing. This proved to have a consistently low coefficient of friction that allowed us to use a small motor of only 300mN force and associated low power consumption, so helping to reduce the size of the product.  Most critically, the exceptionally low static friction [stick/slip effect] gave the smooth operation we needed for fast and accurate focusing.”

A second critical requirement was the temperature range of the bearing.  Thermal imaging products must work effectively in high temperature environments; Fluke specified 50°C ambient.  Taking into consideration a possible 25°C rise due to heat dissipation inside the unit, the bearing had to be capable of performing at temperatures close to 75°C.

At the other end of the scale, storage temperatures can be as low as -20°C giving an overall range of almost 100°C, posing a serious challenge for the grease used in the bearing assembly.

As well as the expected increase in friction at lower temperatures due to thicker grease, tests also revealed that friction varied significantly depending on the amount of grease in the bearing. Fluke tested a number of different lubricants and developed a rig for injecting exactly the right quantity into the bearing housing.

For the large numbers of bearings involved, removing and replacing the standard grease was not a realistic option, so Thomson worked with Fluke to incorporate the special grease in production using a refined version of the Fluke jig.

Benefits of customisation
It took a custom, stainless steel and polymer corrosion-resistant compact bearing from Thomson to meet Fluke’s special requirements. Brian Gattman again:

“All through the development phase and into pre-production, we and Thomson worked together very effectively and, starting late 2013, with the Ti400 range now in full production, deliveries have been on target and to specification. Close cooperation between our two organisations was a major factor in the success of this project.”

Customisation by Thomson of the miniature linear ball bushing bearings enabled Fluke to find the optimum linear guidance system for its thermal imaging camera. The standard Thomson bearings’ small size, low friction and increased accuracy were already attractive to Fluke and it took only minor modification by Thomson to meet Fluke’s particular requirements for the Ti400 instrument.

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