Sunday, August 22, 2010

Roll Pins

Roll pins (aka 'rolled spring pins' or 'slotted spring tension pins') are a very good thing to have on hand. They come in a wide range of sizes; the two pictured give an idea. The large one is 3/8" diameter x 2" long. The small one is 3/32" diameter x 5/8" long. (There are metric dimensioned ones too, of course.)

The two shown here are from a 120-piece assortment I got from Princess Auto. Should I ever need a size or a quantity that the assortment doesn't provide for, I'll have to go to an industrial supply outfit like Spaenaur and buy a package quantity (typically 100 pieces).

The pins are made from quite hard spring steel, rolled into shape as the name implies. The key feature of them is the dimensioning. A roll pin's diameter as you receive it is always slightly oversize of nominal; e.g. a 1/8" (0.125") diameter roll pin will have an actual diameter of 0.131" to 0.135". To install one, drill a hole the nominal size and hammer in the pin. (One or both ends is/are always chamfered for ease of starting.) As the pin is driven, it contracts to fit the hole and effectively gives an interference fit of considerable strength.

An installed pin can be punched out and reused, which means that salvage can be a source of them. (A word of caution about punching them out: be sure to use an exact-size punch, and be especially careful if you're starting on a slightly protruding end of a pin. An undersize punch that manages to get down into the hollow centre of the pin can make a nasty mess of things. There are special punches available that have a centring point on them. You'd have to go to Spaenaur or a similar place for those; they're not something you're likely to find at the Home Depot.)

Uses are endless -- pinning gears or virtually anything to shafts; reinforcing glued up cylindrical assemblies; as motion-limiting stops in adjustable assemblies; what-have-you. Uses have a way of suggesting themselves when you have the pins at hand.

Occasionally, the hollow centre can be an undesirable feature; e.g. on a fertilizer spreader axle where corrosive material could get inside. In such cases, the centre can be filled with a length of soft steel wire or rod and epoxy or CA adhesive.

In closing, I must leave you with a word of caution about Princess Auto's assortment; the smaller diameter pins are poorly formed and seriously oversize, and will not install correctly in nominal size holes. To use them successfully, you have to mike them and select a suitable number size drill. For example, the 3/32" (0.09375") pins are about 0.113" in diameter. That's almost 0.020" oversize, and that's a lot for an interference fit. A No. 38 drill (0.1015" diameter) worked ok.



Here's some information you may find helpful for dealing with oversize fractional inch diameter roll pins. Following is a list of all the available nominal diameters with some added information. Each entry is in the form:

Nominal Fractional Diameter; Nominal Decimal Diameter; Maximum Allowed Decimal Diameter*; Difference (Difference Rounded Off to the Nearest Thou)

* i.e. the maximum diameter that a given pin should have from the factory. A pin whose diameter exceeds this figure will be troublesome to install in a nominal-size hole.

1/16"; 0.0625"; 0.069"; 0.0065" (0.007")

5/64"; 0.078125"; 0.086"; 0.007875" (0.008")

3/32"; 0.09375; 0.103"; 0.00925" (0.009")

1/8"; 0.125"; 0.135"; 0.010" (0.010")

5/32"; 0.15625"; 0.167"; 0.01075" (0.011")

3/16"; 0.1875"; 0.199"; 0.0115" (0.12")

7/32"; 0.21875"; 0.232"; 0.01325" (0.013")

1/4"; 0.250"; 0.264"; 0.014" (0.014")

5/16"; 0.3125"; 0.330"; 0.0175" (0.018")

3/8"; 0.375"; 0.395"; 0.020"; (0.020")

1/2"; 0.500"; 0.524"; 0.024" (0.024")

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SUNDAY, JULY 3, 2011

Addendum -- Application Examples

Here's an example of a pin anchoring a stud at the end of a rod.

And here's a roll pin serving as a valve handle.

Scroll down in either and you'll get to it.

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