Smiths AT speedometer
The Smiths AT speedometer is as Smiths described it 'a heavy duty magnetic type speedometer it has the same basic principles as the Nemag type, but the design is radically different'. This is true in all elements of the statement. The Smiths AT is a far more robust instrument than is the normal Nemag, and so it should be, as it was intended to be fitted to; trucks tractors and other larger vehicles. They were fitted in some early Bentleys and Rolls Royce. In trucks they were fitted through the alphabet of trucks from Albion to Scammell and a variety of coaches. It is possibly that they may predate the mark one Nemag However, at present I cannot confirm this. The earliest one I have work on came from a 1923 3Litre Bentley I am tempted to think it may have been fitted later.
These seem to have been a few different designs all following the same basic principle. If we were to consider the Nemag as a horizontal design the AT is perhaps best considered as a vertical design. The aluminium alloy casing is a far more robust design and of far better quality than the smiths governor it does not seem to suffer from the Smiths Governor's exfoliation problems!
As in the Nemag there is a magnet driven from the normal flexible speedometer cable, As the magnet assembly revolves, it draws round, by the eddy current principle, an aluminium drag cup assembly. Mounted on this assembly is the pointer or needle, as the speedometer revs increase, the needle moves around the dial. There is no direct mechanical connection from the road wheels to the needle or pointer assembly.
The magnet which is usually fairly strong is housed in the large threaded brass 'cup' shown at the top of the photograph (right) The photograph shows new bearings and other replacement parts.
The main shaft is of brass running in two fairly substantial but internally exposed ball bearing races. These do wear, and can cause the central shaft which carries the magnet assembly to come into contact with the drag cup, either at best giving a very erratic output at the needle, or destroying the drag cup assembly entirely.
In addition, the side shake of the worn shaft, can cause damage to the tufnol fibre gear which is driven from a thread machined in the side of the rotating brass shaft. Damage to the gear can result in damage to the odometer drive system. The very small internal gears of the odometer are also prone, after time, to damage.
Most of the problems encountered with these speedometers can, with care, be repaired. I can fit new enclosed roller bearings, make new odometer drive gears (now in Delrin), and in some cases replace the small internal odometer gears. I have recently after catastrophic damage to a speedometer re-made a whole new central shaft assembly.
The earlier AT's seem to have a aluminium alloy spider which held the drag cup assembly to the main housing, in later speedometers this was replaced with a much stronger aluminium alloy housing screwed to the main housing.
Photograph (Below) showing a very damaged hair spring fitted to an earlier aluminium spider.
The drag cup can been seen it is supported from a single 'foot' also damaged.
Photograph (below) A much later and much more robust assembly, the drag cup arrangement is still broadly similar.
Smiths AT 3 Litre Bentley speedometer after full rebuild and lots of repairs, now on calibration test running steadily at 60mph