Re: Fogging Tachometer / Hour Meter

Posted by dgoddard on October 16, 2015 at 20:31:06 from (104.245.136.182):

In Reply to: Fogging Tachometer / Hour Meter posted by dgoddard on October 13, 2015 at 15:03:03:

Quoting Removed, click Modern View to seeThanks for the suggestion. I think I may use a variation of it. I have made a vacuum chamber out of 3" PVC pipe and a little experimentation with my compressed air driven Bernoulli effect vacuum pump draws a vacuum so quickly I was afraid that it might pop out the lens. However a small vent hole would allow rapid equalization of the pressure on the unit. The hole can be easily sealed with epoxy after the unit is dried out or a small screw inserted with sealant. I will try to get some pictures but I cannot get back to the project until Saturday evening.

For the record the Tach/hour meter is a permanently assembled unit. The body is a "can" with an open end at the lens end and a hole at back end where bearing housing sticks out to allow the attachment of the flexible drive shaft. Apparently the mechanism is inserted through the open end and then the bezel which holds the lens it crimped on and forms part of a flange. The unit is installed by inserting it into the instrument panel from the operator's side, and then a bracket on the back side of the panel is bolted on to hold it into the panel.

Where the bearing housing comes through the back of the housing "can" a generous amount of some green sealant has been applied. So apparently it is intended to be a sealed unit. I may consider using a hot glue gun to seal the lens and the bezel so that I do not have to wait for silicone to dry and also to seal any vent hole I drill.

I decided to go ahead and clean and re-grease the flexible shaft and while removing it from the side of the engine (it is driven at the front of the engine) I saw a small piece of something fall on the shop floor. I thought it was a tiny rubber plug but when I picked it up it turned out to be a tiny magnet. It is surprisingly strong it is cylindrical a and measures 5mm in diameter by 5 mm long (5mm = 0.197 " or about 3/16"). Careful examination of the shaft and the instrument indicates it is not part of them and was probably something hanging on the wiring support bracket that holds the flex shaft housing. I have no idea what it might belong to, and just for the record I did verify the the shaft & meter assembly function properly and are missing no parts. So if anyone has an idea what this mystery magnet is please share.

At lens diameter of 2 inches if I could get a perfect vacuum, 1 atmosphere of pressure would be 14.7 psi times an area of 3.14 square inches or 46 lbs of force so fracturing or popping the lens out if I do not vent the can is a real possibility.

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