How an Electric Tyre Pump Works
Twelve Volt electric motor "A" turns at high speed and a small toothed wheel on its spindle drives a larger plastic toothed wheel "B" at a slower speed. This is fixed to a cranked spindle which rotates and pushes/pulls rod "C" that is connected to a piston inside cylinder "D". On the downstroke, air is forced out of outlet "E" and the average pressure is indicated by dial gauge "F", whose accuracy is not likely to be good.
The whole assembly can be manufactured very cheaply by using Mazac (zinc alloy) metal castings. The piston displacement is quite small - no more than two cubic centimetres - and the actual pressure achievable is around 60 psi. Nowhere near the claimed "250 - 300 psi" but just about enough to pump up an ordinary bicycle tyre. Pumping up a car tyre takes a long time because of the small capacity of the cylinder.
This type of pump may be useful as an emergency aid to top up a leaking tyre but is usually unsuitable for use on a frequent basis. Don't expect to achieve the recommended tyre pressure. It's merely a "get you to the nearest garage" aid.
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