How to Make a Strength-Testing Machine
The penny-in-the-slot strength-testing machine is popular among men and boys, presumably because many of them like to show other people what their muscles are capable of, and the opportunity of proving it on a graduated dial is therefore tempting, especially if there be a possibility of recovering the penny by an unusually good performance.
For the expenditure of quite a small number of pence, one may construct a machine which will show fairly accurately what is the value of one's grip and the twisting, power of the arms; and, even if inaccurate, will serve for competitive purposes. The apparatus is very simple in principle, consisting of but five pieces of wood, an ordinary spring balance registering up to 40 lbs., and a couple of handles.
The total cost is but a couple of shillings at the outside.
Fig. 162 is a plan of the machine as used for grip measuring. The base is a piece of deal 1 inch thick, 2 feet long, and 5-1/2 inches wide. The lever, L, is pivoted at P, attached to a spring balance at Q, and subjected to the pull of the hand at a point, R.
The pressure exerted at R is to that registered at Q as the distance PQ is to the distance PR. As the spring balance will not record beyond 40 lbs., the ratio of PQ to PR may conveniently be made 5 to 1, as this will allow for the performances of quite a strong man; but even if the ratio be lowered to 4 to 1, few readers will stretch the balance to its limit.
The balance should preferably be of the type shown in Fig. 162, having an indicator projecting at right angles to the scale through a slot, as this can be very easily fitted with a sliding index, I, in the form of a 1/4-inch strip of tin bent over at the ends to embrace the edges of the balance.
As the pressures on the machine are high, the construction must be solid throughout. The lever frame,
FIG. 162. - Plan of strength tester.
A, and pivot piece, C, should be of one-inch oak, and the two last be screwed very securely to the baseboard. The shape of A is shown in Fig. 163. The inside is cut out with a pad saw, a square notch being formed at the back for the lever to move in. The handles of an old rubber chest expander come
FIG. 163. - Grips of strength tester.
in useful for the grips. One grip, D, is used entire for attachment to the lever; while of the other only the wooden part is required, to be mounted on a 1/4-inch steel bar running through the arms of A near the ends of the horns. If a handle of this kind is not available for D, one may substitute for it a piece of metal tubing of not less than 1/2- inch diameter, or a 3/4-inch wooden rod, attached to an eye on the lever by a wire passing through its centre.
A handle, if used, is joined to the lever by means of a brass plate 3/4 inch wide and a couple of inches long. A hole is bored in the centre somewhat smaller than the knob to which the rubber was fastened, and joined up to one long edge by a couple of saw cuts. Two holes for good-sized screws must also be drilled and countersunk, and a socket for the knob must be scooped out of the lever. After making screw holes in the proper positions, pass the shank of the knob through the slot in the plate, and screw the plate on the lever. This method holds the handle firmly while allowing it to move freely.
The lever tapers from 1-1/2 inches at the pivot to 5/8 inch at the balance end. The hole for the pivot-- 5/16-inch steel bar--should be long enough to admit a piece of tubing fitting the bar, to diminish friction, and an important point, be drilled near the handle edge of the lever, so as to leave plenty of wood to take the strain. The last remark also applies to the hole for the balance pin at Q.
The balance support, B, and the pivot piece, C, are 2-1/2 and 2- 7/16 inches high respectively. Run a hole vertically through C and the baseboard for the pivot, which should be 4-1/2 inches long, so as to project 1 inch when driven right home. Take some trouble over getting the holes in L and C quite square to the baseboard, as any inaccuracy will make the lever twist as it moves. To prevent the pivot cutting into the wood, screw to the top of C a brass plate bored to fit the pivot accurately. The strain will then be shared by the screws.
The horns of A should be long enough to allow the outside of the fixed grip to be 2-1/4 inches from the inside of the handle.
The balance is secured first to the lever by a pin driven through the eye of the hook, and then to B by a 3-inch screw passed through the ring. The balance should just not be in tension.
When the apparatus is so far complete, test it by means of a second balance applied to D. Set the scale-marker at zero, and pull on the D balance till, say, 35 lbs. is attained. If the fixed balance shows 7 lbs. on what is meant to be a 5 to 1 ratio, the setting of R relatively to P and Q is correct. If, however, there is a serious discrepancy, it would be worth while making tests with a very strong balance, and establishing a corrected gradation on a paper dial pasted to the face of E.
For twisting tests we need a special handle (see Fig. 164), which is slipped on to the pivot and transmits the twist to L through a pin pressing on the back of the lever. The stirrup is made out of strip iron, bent to shape and drilled near the ends for the grip spindle. To the bottom is screwed and soldered a brass or iron plate, into the underside of which the pin is driven.
FIG. 164. - Handle for twisting test.
To prevent the handle bending over, solder round the pivot hole 3/4 inch of brass tubing, fitting the pivot closely.
Tests. - Grip tests should be made with each hand separately. The baseboard should lie flat on a table or other convenient support, and be steadied, but not pushed, by the hand not gripping.
Twisting tests may be made inwards with the right hand, and back-handedly with the left. The apparatus is stood on edge, square to the performer, resting on the horns of A and a support near the balance.
Finger tests are made by placing the thumb on the front face of B, and two fingers on the farther side of the lever, one to the left and the other to the right of the tail of the balance.