An Introduction to Machine Drawing and Design Part 3

IV. KEYS.

_Keys_ are wedges, generally rectangular in section, but sometimes circular; they are made of wrought iron or steel, and are used for securing wheels, pulleys, cranks, &c., to shafts.

[Ill.u.s.tration: Fig. 20.]

Various sections of keys are shown in fig. 20. At (_a_) is the _hollow_ or _saddle key_. With this form of key it is not necessary to cut the shaft in any way, but its holding power is small, and it is therefore only used for light work. At (_b_) is the _key on a flat_, sometimes called a _flat key_. The holding power of this key is much greater than that of the saddle key. At (_c_) is the _sunk key_, a very secure and very common form.

The part of the shaft upon which a key rests is called the _key bed_ or _key way_, and the recess in the boss of the wheel or pulley into which the key fits is called the _key way_; both are also called _key seats_.

With saddle, flat, and sunk keys the key bed is parallel to the axis of the shaft; but the key way is deeper at one end than the other to accommodate the taper of the key. The sides of the key are parallel.

The _round key_ or taper pin shown at (_d_) is in general only used for wheels or cranks which have been previously shrunk on to their shafts or forced on by great pressure. After the wheel or crank has been shrunk on, a hole is drilled, half into the shaft and half into the wheel or crank, to receive the pin.

When the point of a key is inaccessible the other end is provided with a _gib head_ as shown at (_e_), to enable the key to be withdrawn.

A _sliding_ or _feather key_ secures a piece to a shaft so far as to prevent the one from rotating without the other, but allows of relative motion in the direction of the axis of the shaft. This form of key has no taper, and it is secured to the piece carried by the shaft, but is made a _sliding fit_ in the key way of the shaft. In one form of feather key the part within the piece carried by the shaft is dovetailed as shown at (_f_). In another form the key has a round projecting pin forged upon it, which enters a corresponding hole as shown at (_g_). The feather key may also be secured to the piece carried by the shaft by means of one or more screws as shown at (_h_). The key way in the shaft is made long enough to permit of the necessary sliding motion.

_Cone Keys._--These are sometimes fitted to pulleys, and are shown in fig. 32, page 38. In this case the eye of the pulley is tapered and is larger than the shaft. The s.p.a.ce between the shaft and the boss of the pulley is filled with three _saddle_ or _cone keys_. These keys are made of cast iron and are all cast together, and before being divided the casting is bored to fit the shaft and turned to fit the eye of the pulley. By this arrangement of keys the same pulley may be fixed on shafts of different diameters by using keys of different thicknesses; also the pulley may be bored out large enough to pa.s.s over any boss which may be forged on the shaft.

_Proportions of Keys._--The following rules are taken from Unwin's 'Machine Design,' pp. 142-43.

Diameter of eye of wheel, or boss of shaft = _d_.

Width of key = 3/4_d_ + 1/8.

Mean thickness of sunk key = 1/8_d_ + 1/8.

" key on flat = 1/16_d_ + 1/16.

The following table gives dimensions agreeing with average practice.

_Dimensions of Keys._

D = diameter of shaft.

B = breadth of key.

T = thickness of sunk key.

T_{1} = thickness of flat key, also = thickness of saddle key. Taper of key 1/8 inch per foot of length, _i.e._ 1 in 96.

+---------------------------------------------------------------+ D 3/4 1 1-1/4 1-1/2 1-3/4 2 2-1/4 2-1/2 +-----+-----+-----+-------+-------+-------+-----+-------+-------+ B 5/16 3/8 7/16 1/2 9/16 5/8 11/16 11/16 T 1/4 1/4 1/4 5/16 5/16 5/16 3/8 3/8 T_{1} 3/16 3/16 3/16 3/16 1/4 1/4 1/4 5/16 +---------------------------------------------------------------+

+-------------------------------------------------------------------+ D 2-3/4 3 3-1/2 4 4-1/2 5 5-1/2 6 +-----+-------+-----+-------+-------+-------+-------+-------+-------+ B 3/4 7/8 1 1-1/8 1-1/4 1-3/8 1-1/2 1-5/8 T 3/8 7/16 1/2 1/2 9/16 5/8 11/16 3/4 T_{1} 5/16 5/16 3/8 7/16 1/2 1/2 9/16 5/8 +-------------------------------------------------------------------+

+-------------------------------------------------------+ D 7 8 9 10 11 12 +-----+-------+-------+-------+--------+--------+-------+ B 1-7/8 2-1/8 2-3/8 2-5/8 2-7/8 3-1/8 T 13/16 15/16 1 1-1/16 1-3/16 1-1/4 T_{1} 11/16 3/4 7/8 15/16 1-1/16 1-1/8 +-------------------------------------------------------+

V. SHAFTING.

Shafting is nearly always cylindrical and made of wrought iron or steel.

Cast iron is rarely used for shafting.

_Axles_ are shafts which are subjected to bending without twisting.

The parts of a shaft or axle which rest upon the bearings or supports are called _journals_, _pivots_, or _collars_.

In journals the supporting pressure is at right angles to the axis of the shaft, while in pivots and collars the pressure is parallel to that axis.

Shafts may be solid or hollow. Hollow shafts are stronger than solid shafts for the same weight of material. Thus a hollow shaft having an external diameter of 10-1/4 inches and an internal diameter of 7 inches would have about the same weight as a solid shaft of the same material 7-1/2 inches in diameter, but the former would have about double the strength of the latter. Hollow shafts are also stiffer and yield less to bending action than solid shafts, which in some cases, as in propeller shafts, is an objection.

VI. SHAFT COUPLINGS.

For convenience of making and handling, shafts used for transmitting power are generally made in lengths not exceeding 30 feet. These lengths are connected by couplings, of which we give several examples.

[Ill.u.s.tration: FIGS. 21 and 22.]

_Solid_, _Box_, or _m.u.f.f Couplings._--One form of box coupling is shown in fig. 21. Here the ends of the shafts to be connected b.u.t.t against one another, meeting at the centre of the box, which is made of cast iron.

The shafts are made to rotate as one by being secured to the box by two wrought-iron or steel keys, both driven from the same end of the box. A clearance s.p.a.ce is left between the head of the forward key and the point of the hind one, to facilitate the driving of them out, as then only one key needs to be started at a time. Sometimes a single key the whole length of the box is used, in which case it is necessary that the key ways in the shafts be of exactly the same depth.

The half-lap coupling, introduced by Sir William Fairbairn, is shown in fig. 22. In this form of box coupling the ends of the shafts overlap within the box. It is evident that one shaft cannot rotate without the other as long as the box remains over the lap. To keep the box in its place it is fitted with a saddle key.

It will be noticed that the lap joint is sloped in such a way as to prevent the two lengths of shaft from being pulled asunder by forces acting in the direction of their length.

Half-lap couplings are not used for shafts above 5 inches in diameter.

It may here be pointed out that the half-lap coupling is expensive to make, and is now not much used.

As shafts are weakened by cutting key ways in them, very often the ends which carry couplings are enlarged in diameter, as shown in fig. 21, by an amount equal to the thickness of the key. An objection to this enlargement is that wheels and pulleys require either that their bosses be bored out large enough to pa.s.s over it, or that they be split into halves, which are bolted together after being placed on the shaft.

_Dimensions of Box Couplings._

D = diameter of shaft.

T = thickness of metal in box.

L = length of box for b.u.t.t coupling.

L_{1} = length of box for lap coupling.

_l_ = length of lap.

D_{1} = diameter of shaft at lap.

+---------------------------------------------------------------+ D 1-1/2 2 2-1/2 3 3-1/2 4 +-------+--------+--------+---------+-------+----------+--------+ T 1-1/8 1-5/16 1-1/2 1-3/4 1-15/16 2-1/8 L 5-3/4 7 8-1/4 9-1/2 10-3/4 12 L_{1} 4-1/8 5-1/4 6-3/8 7-1/2 8-5/8 9-3/4 _l_ 1-7/16 1-7/8 2-5/16 2-3/4 3-3/16 3-5/8 D_{2} 2-5/16 3 3-11/16 4-3/8 5-1/16 5-3/4 +---------------------------------------------------------------+

+----------------------------------------------+ D 4-1/2 5 5-1/2 6 +-------+---------+--------+--------+----------+ T 2-5/16 2-1/2 2-3/4 2-15/16 L 13-1/4 14-1/2 15-3/4 17 L_{1} 10-7/8 12 -- -- _l_ 4-1/16 4-1/2 -- -- D_{2} 6-7/16 7-1/8 -- -- +----------------------------------------------+

Slope of lap 1 in 12.