EW oil hand pump

[RRinOz]

Using Chris' drawing of the hand oil pump  https://www.douglasmotorcycles.net/aa-files/images/chris/2006/Douglas-Oil-Pump-1000.jpg  I found a spring that appears to be almost exactly right. Chris specified a spring of 14 coils made of 0.078 wire and 7/8 inch diameter and 5.5 inches long. RS Components (an electrical/industrial supplier) stock a spring of 2mm wire, 22mm diameter and 135mm long (stock number 121-286). It has about 13 coils and has ground ends. Ideal I thought. The force exerted by the spring when compressed to 2 inches long is approximately 16 kgs. This seems a bit high - I can scarcely pull it up with 2 fingers. Where have I gone wrong? Ralph

[RRinOz]

I have solved the problem by winding a spring of 1.6mm wire. The force required to lift the plunger is now more reasonable and the sight glass shows a steady flow of drops. Now that I have oil I can move onto the engine.

[Doug]

There are quite a number of places on the internet to find the formula for calculating the rate for coil springs. A coil spring is primarily a torsional spring, wound up in a coil. As such wire diameter and number of coils has a big impact on the rate. On a 7/8 diameter spring, a single extra coil is adding about 2.75 inches to the length of the torsion bar, making it more supple. If you did the calculations it would probably surprise you the difference in spring rates (and this is assuming both springs are of the same material).

-Doug

[RRinOz]

Gidday Doug, I was aware of the Hooke's Law stuff but I couldn't see why the spring needed to be so strong (16kg onto a 1 inch diameter cup washer = approx. 45PSI of oil pressure). I've used 1/16 inch music wire (AISI 1086) and wound it on my lathe set to its maximum thread pitch of 4 TPI. Result is a 22 turn spring giving 2.5kg at 2 inches long or 7PSI of oil pressure. Only potential problem is that at maximum length, the oil flow rate may drop off a bit too much. That is to say that this spring may not be my last! Thanks for your interest, Ralph.