8/2/2019 2847-Keys & Driving Pins

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Keys and driving pinsGEOMETER describes a number of ways of fixing

sprockets and similar devices to shafts

0 TRANSMIT DRIVE to or froma shaft via a pulley, sprocket,g e a r o r h u b , me a n s a r e

necessary to take normal turningtorque without slip or relativemovement. Usually, it is accom-plished through keys or driving

pins when components cannot bepermanently fixed and it is neces-sary to allow for assembling anddismantling.

Three common types of keys areemployed-the gib or taper key, theparallel key and the Woodruff key.

The gib key A used for industrialand agricultural machinery, has ahead for fitting and extracting andwhile parallel on the sides is taperedtop and bottom for driving firmly intothe keywayed boss of the pulley orwheel. For this key, a slot or way is

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cut from the end of the shaft alongwhich it is tapped. The boss of thepulley or wheel may be circular, or tosave metal and provide strength mayhave extra metal in the region of thekeyway.

The parallel key is almost exclus-ively employed where the hub isattached to the shaft by a long taper---of which examples occur on cars onaxle shafts. There are rounded endsto the key, which fits in a milled slotin the taper, the hub being taperedand keywayed to suit.

The common key for short taperand parallel fittings for gears, sprocketsand pulleys, in all sorts of car andmotorcycle applications, is the Wood-ruff type, B and C. This is in the formof part of a disc of steel fitting in arecess in the shaft which has beenmilled with a key-seating cutter.

Fitting and removingTo prevent movement, all keys

should fit well in shafts and hubs.Gib keys should be a sliding fit inboth. Parallel and Woodruff keysshould be a light tapping fit in shafts,a sliding fit in hubs. On occasion,gib and parallel keys can be filed fromsquare or flat steel; Woodruff keysmade by machining or sawing discsfrom round bars.

To fit gib keys, pulleys or wheelsshould be positioned on shafts, thenthe keys lightly driven, noting there isno butting up of heads. To remove,an extractor as shown can be used.If there is space, pulleys and wheelscan be driven along shafts shouldthe keys be tight and refuse to move.

To fit Woodruff keys, they aretapped into shafts with a downwardinclination at the front, for matingkeyways to engage easily. To remove,they are lifted out with a screwdriver,carefully punched upwards, or drawnwith split-pin pliers. Burrs are filed off.

As substitutes for keys in modelwork and light drives, pins are usedin various ways-particularly where

shafts have shoulders with hubspulled tight by nuts.With the boss up to the shoulder,

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an undersize hole is drilled downabutting faces and a pin driven orscrewed in, D. If necessary, the half-circle in the boss is cleared with around file.

Alternatively, the pin is fitted rightthrough the shaft, in front of theshoulder, E, engaging slots being filed

in the boss.A pin may be prevented from comingout by fitting at an angle, F, and aface fitting arranged as G. For drillingthe latter, a washer can be used as atemplate on the shaft, then locatedon a rod in the boss for drilling this.A stronger multiple-pin drive can beprovided using a washer with moreguide holes, four for example, H .

Semi-permanent fittings for collars,etc., on shafts can be arranged bydrilling both right through and usinga taper pin-or a parallel pin rivetedinto countersinks each side. Again,

if the collars are at the ends of shafts,holes can be drilled and tapped halfin one, half in the other, I.

M O D E L E N G I N E E R