VDO Oil Pressure Gauge repair

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This is a quick guide to show the internal guts of a Bourdon type Pressure gauge and what I have done to not only fix but make much more robust with respect to vibration induced wear.

[Note needles also bounce due to oil pressure pulsation; that can be usually minimized by using an accumulator type device at the input to the gauge. Basically you need a relatively large cavity of hose and a small bleed port to fed the gauge.While this stops needle bounce (an probably bounce related gauge wear) it does nothing to stop vibration from rattling the mechanism which will lead to pin wear.]


If you have even installed a pressure gauge on your bike, and especially onto your engine you probably have noticed that while it might have worked pretty well initially after a while it is not so good. Well the mysteries of what is going on are soon unlocked if you look inside the gauge.

Here you can see that I used the old double flat head screwdriver trick to pry the lip back on the gauge Bessel cover ring. This is a method popularized fro opening and repairing tachs and speedo gears. I would not recommend the other technique of cutting this metal gauge with a dremel.






With a little paitence, it come apart just like the other gauges.



Here is the mechanism that you will fin inside. This is what wears out causing zero shifts and under indication of oil pressure.




You can read about it here.

http://en.wikipedia.org/wiki/Pressure_measurement

Bourdon[edit]

The Bourdon pressure gauge uses the principle that a flattened tube tends to straighten or regain its circular form in cross-section when pressurized. Although this change in cross-section may be hardly noticeable, and thus involving moderate stresses within the elastic range of easily workable materials, the strain of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is pressurized. Eugene Bourdon patented his gauge in France in 1849, and it was widely adopted because of its superior sensitivity, linearity, and accuracy; Edward Ashcroft purchased Bourdon's American patent rights in 1852 and became a major manufacturer of gauges. Also in 1849, Bernard Schaeffer in Magdeburg, Germany patented a successful diaphragm (see below) pressure gauge, which, together with the Bourdon gauge, revolutionized pressure measurement in industry.[6] But in 1875 after Bourdon's patents expired, his company Schaeffer and Budenberg also manufactured Bourdon tube gauges.

Basically what wears is the parts that join 3,4, and 5. These are little hinges that converts the movement of the end of the flattened tube to rotating motion of the level (part 5) which is what actually rotates the needle.

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This is what the VDO looks like , and it is virtually unchanged from the original designs. The problem is that the pivots are made very small to reduce friction
But this is actually a fallacy, as the design was borrowed from watch making which may very well need friction less mechanisms to avoid slowing the watch, but with the available pressure from the liquid, the same problem just doesn't exist.

The solution? Make the pivots more robust by using a larger diameter tube.



Here are the disassembled parts. You can tel from the straight pin, that the open hole is elongated from vibration. This causes a big deadband in the movement of the needle. You can also see the end of the pin that the gear arm rides on. The other side was actually completely warn off.





So I used some brass tube stock from the hobby shop and cut out some increased diameter bearings and a new pivot for the main gear arm. I had to carefully drill the pivot holes to accept the new pivots. You can see the solder on the main bearing for the gear arm.









The unit goes back together pretty easily. The only thing to watch is where to re mesh the gears so that the needle is a zero with zero pressure.

One detail, though is that there is how to retain the pivots which are not part of the gear arm without soldering them. I figured if I got any solder on them the solder would flow into the bearing surfaces and ruin the smooth action of the joint. What I did instead was arrived at what I think is a suitable solution that kills two birds with one stone.

If you notice there is a dab of black material on the pivot. This is liquid tape commonly used to insulate wire. The liquid tape secured the end of the pin keeping it from slipping out, but it also provides a shock absorber of sorts. Even if this much larger pin starts to wallow and the pivot beats against the arm, the rubber will support the joint (kind of like cartridge in your knee) and keep the pin from beating against it.. Basically pivot is now rubber mounted. The movement is so small that the rubber easily flexes.

I thought about patenting this, but alas to many other problems to solve rather than 150 year old problems applied to 30 year old bikes. Now if I could figure out how to involve cloud computing some how:???







After reassembly I did a pressure check with another pressure gauge I had. I will not say it was perfect as the other gauge was a cheap HF pressure gauge. However the gauge seems to respond well to pressure build up and did not exhibit any zero offset effects no and now pressure non linearity. The main place where the two gauges diverged was in the upper scale area of 10 psi and in this area I'm looking more at the intrinsic accuracy of the tube rather than the rework of the movement.

I replaced the cover and replaced the gauge on my handlebars and it has been working well since last year (may 2013)

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Reserved ........