Chapter 18

 

Chapter 18

Motors and Fixings: Joining rubber, tensioning, lubrication, winding, counters.

WE have already considered the use of the rubber motor, and we know that it is in one or more skeins, but we want to know now how to make up the skeins. We shall have to experiment to find out how much rubber is needed for a particular model, in the manner described for trimming the model for flight.

However, having decided on a certain quantity to try, we join the two ends together. There seems to be two methods of joining, both used satisfactorily by the writers. One is to tie a reef knot and bind the ends. The other method is to lap the two ends of the rubber together with about half an inch overlap, and then bind them together, using thread or "Sylko", the rubber being stretched during the binding.

To assist us in this process, a gadget devised by Mr. R. Colman is shown in Fig. 61.

A start is made with a base 5 1/2 in. x 3 in. of medium hard wood. In this are cut two rectangular holes 1 1/16in. x 17/32in., these being 1/2in. apart and with their longer sides parallel.

Fig.61Fig.61

Four pegs of 1/2in. square hard wood, each 2 in. long, are now cut and paired off: - A, B, C and D. One inch from the top of each peg drill a hole a 5/32in. in A and C, and a 7/32in. in B and D. Through A and C insert 3/16in. diameter bolts 1 1/2in. long (these being purchased with wing nuts to fit), which are, intentionally, tight fits, to prevent them turning when the wing nuts are tightened. The bolts will be an easy fit in pegs B and D, which must have some movement. To prevent damage to the rubber strip, the top of each peg is bound with thin sheet rubber (bicycle or motor-car inner tube), this being secured where shown, with short pins. The pairs of pegs are next inserted in the base holes.

To operate: Slacken the wing nuts whilst keeping the pegs in the holes, and place the ends of the rubber to be tied between pegs C and D, afterwards tightening the nut. Stretch the rubber and bring it between pegs A and B, and tighten the nut. To obtain the necessary extra tension before tying, a hardwood key (3 in. long x 1/2in. wide and 1/4in. thick, with a 2 in. length of 3/16in. birch dowel inserted for turning) is placed between the two pairs of pegs and given a quarter turn, thus pushing the pegs and the rubber further apart. The rubber is best tied with best quality silk, which is taken round the strip three times before tying. A touch of rubber solution to the short ends will make a permanent and non-slippable joint. This joining, by the way, should be done before lubricating the rubber.

We now loop the rubber into the number of strands required, and it is a good idea to put a bobbin on each end, held with a rubber band, stretched and wound on. The length of the rubber is often greater than the distance between the hooks, and we have already seen how to keep it taut by means of a fitting on the propeller shaft, but there is another method of doing this in the rubber itself. The rubber is wound so that it twists itself up into a shorter length.

Fig.62, Fig.63Fig.62, Fig.63

The simplest way to do this is to make up the motor into twice the length, but with only half the number of turns. In the centre of this we tie a piece of cotton, or fix a bobbin, so that we shall know the exact middle when the rubber is wound up. We attach one end to a fixed hook and the other to a winder of some sort (A in Fig. 62). It is then wound up a few turns. The best number will have to be found by experiment, but you might try about 50 to start with. Fold the wound skein into two, so that you have the correct number of strands for the motor, and put the two ends on a hook or a bobbin. Stretch the skein and then release it, letting it wind itself up, and see if the length is slightly less than the distance between the hooks in the model. If so, all is well, but if it is a bit slack, unhook the ends and wind on a few more turns. If it is a bit on the tight side, try a few less turns. Another method is to wind each loop separately, and so it is equally successful with an odd number of loops. In this case a special gadget has been made for winding and testing rubber motors. This is shown in Fig. 63, and we will let Mr. S. E. Capps, the originator, describe it.

The baseboard A is prepared first from a piece of straight-grained deal or pine (any wood will do, provided one is able to work it properly) about 4 ft. long, planed smooth, and

the ends carefully squared. The slot is next cut, and is 3/8in. to 1/2in. wide. This is easily done by first boring a hole at each end and cutting down with a fine saw. Carefully mark off and drill all the holes at both ends. The two cleats, B, should now be made and fitted in place, after which the whole board should be sandpapered all over with No. 0 or No. 1 sandpaper, to remove all sharp corners and edges. This is important, as any sharp edge or corner coming in contact with the rubber would probably cause a small cut, which would eventually result in the fracture of the motor.

Next cut the four diagonal pieces, C, for the sides of head and tail parts. These should be just as carefully made. Cut the two bottom plates, E, and drill the holes shown. These

can now be screwed together, and when completed should be smoothed in the same way as the board.

The head or fixed part can now be fitted and fixed to the board with a round-headed bolt. The final parts to be made are the formers, D, to take the nose and tail plugs, G, of the model whose motor is to be tested. These are made of three-ply wood, and after being smoothed are fixed with small woodserews in the corners. The whole may then be varnished or enamelled.

To use this gadget for tensioning our motor, we put a hook in the tail part and two hooks in the nose part. Refer again to Fig. 63. We put one end of the rubber motor on the tail hook and the other end on hook A at the front. Taking each loop in turn, we wind it up a few turns and transfer it to hook B. We wind them in the same direction as the propeller revolves. When all are wound we put them on the nose of the model, holding the propeller to prevent it turning, and then put the nose into place on the gadget and release the propeller. The rubber will then twist itself up into a sort of rope. If we move the tail anchorage forward till the distance between the two hooks is the same as it will be in the model, we shall see if the loops have been wound up the right amount.

We have not so far said anything about lubricating the rubber motor, which should be done before winding, so let us see what it is all about.

We know that where two surfaces slide against each other they are lubricated to reduce friction and prevent undue wear. For the bearings of engines we use oil, and for a dance floor french chalk is used. For our rubber motors we use a special rubber lubricant. It should be well rubbed into the strands of the motor, so that when two pieces are rubbed together they feel very slippery. Lubricate the rubber evenly and freely.

When putting the model away for a few days, all the lubricant should be washed off the rubber, the rubber dried and stored in an air-tight tin. A little french chalk in the tin also helps to preserve the rubber.

The next consideration is fixing the rubber in the fuselage. This, of course, applies to the rear end, since the front will be attached to the nose-block or gearbox. We can either fix it on a hook fixed to the rear end of the fuselage, or use some form of motor stick. Let us consider the motor stick.

Fig.64Fig.64

This is an arrangement whereby the nose-block and rear hook are fastened to a rod or "stick" so that all tension and torsion of the motor is taken by the stick. There is then no strain on the fuselage from the rubber. Also we can arrange it so that the motor is wound up before it is put in the fuselage, so that, should we be unfortunate enough to have the rubber break, the model will not be damaged.

Such a motor stick is shown in Fig. 64. It consists of a square or oblong stick of balsa with a hook at the rear and a nose-piece at the front. The rear hook can be made of 18 or 20 gauge wire, bent to form a hook at the top, then bent round under the stick at the bottom and bound and glued in place. A gusset of ply or balsa is glued on to hold the wire upright. At the front the nose-block is also glued on with a gusset. We shall then need some means of keeping the nose on to the front of the .fuselage. This can conveniently be a hook fastened to a longeron or cross strut an inch or two down the fuselage, connected by a rubber band to a hook at the bottom of the nose-piece and under the stick.

An improved type of motor stick can be made in the form of a tube, which encloses the whole of the motor. This tube is quite a good idea when we have a fuselage with lots of thin stringers, since it prevents the rubber hitting them. Also it is an easy matter to fit a small nose-block that will easily knock out on landing, and so help to save the propeller from damage. The front end of a motor tube is shown in Fig. 65. Here it is glued to a nose plate that fit's up against the front of the fuselage in the same way that a nose-block would fit. In this plate we can have a small removable nose-block. For small models we can make the tube from 1/64 in. sheet balsa covered with paper, and for large models we can use 1/32in. balsa covered with silk. To make the tube we soak the balsa in boiling water - in the copper on wash-day is a good idea - until it is thoroughly hot. Then we borrow the broom (since we used the copper!) and wrap the balsa round the handle. When it is dry we can lap the edges about 1/8in. and glue them together.

Fig.65, Fig.66Fig.65, Fig.66

We want to make the tube as large as we can get in the fuselage, to give as much room as possible to the rubber. For the rear end of the tube we make a disc of balsa from two pieces glued together with the grain crossing, with a large hole to pull the rubber through. To hold the rubber we can use a peg of cane or bamboo, and, to prevent it turning, four blocks of balsa can be glued on as shown in Fig. 66. If we are using two gears, it would be best to have two holes in the back. With this type of motor tube we can wind up the motor inside the fuselage without fear of damage if the motor should break. A motor stick, or tube, has one disadvantage for scale models, and that is the difficulty of adjusting the position of the rear hook to assist in getting the C.G. in the right place. The only thing to do is to make, the stick long in the first place and cut it shorter bit by bit until we get it right.

If we do not use a motor stick we shall have to fix up some means of attaching the motor to the rear of the fuselage. If we turn again to the drawing of the Miles Trainer, we can see two pegs passing through the fuselage from top to bottom, one at the bottom of the leading edge of the rudder and the other just behind the windows. The ends are supported in large blocks of balsa. These two pegs provide two alternative positions for the motor, which is hooked on to a paper tube. To the middle of the tube is fixed a balsa handle for holding the tube in place while we put the peg through. There is a little sketch of it just above the fuselage, in front of the rudder. The tail-plane and rudder are made detachable, so that we can put the tube in from the rear. This is an excellent method of adjustment.

We must remember that there may be a strong pull on the rubber when it is wound up, and there may also be some tendency to twist the fuselage, so where the motor is fixed we must strengthen the fuselage. Small sheets of balsa covering a number of stringers, or across a pair of longerons, are useful here.

Fig.67Fig.67

Another method for varying the position of the rubber motor in the fuselage, a neat fitting, is shown in Fig. 67. The ply bulkhead should be fixed in the fuselage about level with, or just behind, the leading edge of the tail-plane, and we need to make the tail-plane removable so that we can get at the bamboo plug. It is usually quite easy to arrange this, as the tail-plane is generally at the top of the fuselage, and can have the rudder fixed on. Unless we have a very small model, it is best to use ply about 1/16in. thick for the bulkhead, with stiffeners 1/8in. square. The bamboo plug can be about 1/8in. diameter. We can make this plug, or any odd bits of small dowelling for that matter, with a very.simple piece of apparatus. All we need is a piece of steel plate about 1/16in. thick, with a series of holes of different sizes in it. If we cannot find anything better we can get a large hinge - a shiny one is best - and put the holes in that. A set of suitable drills can be obtained for 6d. Need we say where? Now, to make the plug or dowel we cut a piece of wood roughly three or four sizes larger than the finished size we want, and push it through one of the larger holes. Then we push the wood through the next hole smaller, and so on until we have got it down to the right size. Don't try to do a length of more than about six inches at a time, and push a very little at a time. If we can it is best to put the plate in a vice and push with one hand and pull with the other.

The next part of the fitting is the sliding block. For this we can use a piece of balsa about 1/4in. square, and put a piece of ply about 1/32in. thick each side, glued on. The length will depend on the distance between the bulkhead and the rear end or sternpost of the fuselage. With the block as far back in the fuselage as it will go, the front end can be about 1/2in. in front of the rear face of the bulkhead. With the block in this position we want a hole through it to take the bamboo plug just behind the bulkhead. From there to the end we drill a series of holes about 1/2in. apart. We can put the wire for the rubber hook through the ply and balsa about 3/16in. behind the front edge of the block. If we use a bobbin on the rubber motor we can cut away the balsa between the ply, and use another bamboo plug to hold the bobbin in place.

The sketch shows two hooks suitable for two skeins of rubber, but two hooks are also useful for one skein. If we put part of the rubber on one hook and part on the other, it helps to prevent the rubber from bunching in the rear of the fuselage. The block should slide fairly easily through the bulkhead, and by putting the plug in different holes we get a very fine adjustment of the rubber, with nothing showing on the outside. If our model should have the tail-plane about half-way down the fuselage we could still use this idea by fixing the tail-plane to the sides of the fuselage and having the top, complete with rudder, to be removable.

For getting this tail-block in position in the fuselage, a rod is needed, since the block will have to be put in from the nose. The writer used a piece of 1/4in. x 1/4in. hard balsa, though spruce or deal would be just as good, and stand rougher treatment, with two pieces of 1/4in. x 1/16in. ply about 5 in. long, held on to one end with rubber bands. This formed a springy sort of fork, into which the tail-block was slipped, complete with motor. The rod was long enough to stick out of the front of the fuselage about 5 in. when the block was in position. The nose-block was held against this end by the tension of the rubber motor. When the dowel was in place in the tail-block it was a simple matter to pull the nose-block off to withdraw the rod.

Fig.68Fig.68

To wind up the rubber is rather a tedious job if done by hand, so we usually resort to some sort of assistance in the form of a geared winder. Fig. 68 shows one type made from

a hand grindstone. This is bought in bits from the sixpenny store, but you do not need the wheel. Instead we have a distance-piece and bar of iron or brass. The distance-piece can be either metal or wood, and could be cut from a cotton reel. The strip of brass or iron can be about 1/2in. wide by 1/16in. or 3/32in. thick, and about six inches long, bent at right-angles at each end, and has a hole in the middle. It is bolted on to the spindle of the grinder, and the two ends should be bound with rubber to prevent damage to the airscrew. The winder is then clamped on to an iron or steel bar about 1/2in. square, pointed at one end and bent at right-angles at the other. This can then be pushed into the ground with the foot. It is a good idea to drill a hole for the propeller shaft in the spindle of the grinder, or solder a piece of tube on.

Another form of winder can be made from a hand drill. A hand drill can be bought from the 6d. stores (in bits), and all that is needed in addition is a hook or prong arrangement. We can make the hook from a piece of wire or a propeller shaft of a fairly heavy gauge, say about 14. For small models this will do very well, but if we have a lot of rubber to deal with it is much safer to solder the hook in a piece of brass tube, or wind on some thickish copper wire and solder it. The idea is to make a larger diameter for the chuck to grip. For. the prong type we can wind two pieces of wire together round a piece of brass tube about 1/8in. diameter, bend them outwards and forwards about 1/4in. from the front end of the tube and solder them on, opposite to each other, toasting fork fashion. The brass tube is then held in the chuck.

Fig.69Fig.69

The hook type winder is shown in Fig. 69 with the addition of a revolution counter designed by Mr. J. Youhill. This counter consists of a block of hard wood clamped to the drill frame with two hook bolts made from 1/8in. or 4 B.A. screws and nuts. Through the wood is a 3/16in. hole, or a brass bush drilled 3/16in. to take the 2 B.A. countershaft. At one end of this shaft we have two nuts locked together, or preferably soldered to the shaft, with holes drilled in all six flat faces of one nut, and short pieces of wire soldered in. Two more nuts and a washer are put on the shaft, which is then threaded through the block. Next we put on a spring washer-and two more nuts. With the wood block on the drill frame, the first two loose nuts are adjusted so that the star-wheel end is in a convenient position for the striker bar. The two nuts are then tightened against each other so that they will not alter their position. The other two nuts are tightened against the spring washer so that the shaft will revolve easily but will not slip round on its own when shaken.

The striker is a piece of wire or a small screw fixed in the large gear wheel so that it will strike each piece of wire projecting from the "star" wheel. A piece of brass plate about 20 s.w.g. is screwed to the wood block so that it projects about 1 3/4in. This plate either has a slot cut in it or has a piece of wire soldered on to form a slot. Poking through this slot is a pointer, which can be a short piece of wire soldered to a nut that screws easily along the shaft. On the extreme end of the shaft is a terminal nut, fixed on by soldering or with a lock nut. This terminal nut is for returning the pointer to zero after winding up. Turns are counted on the winder hook and the brass plate is marked to suit.

An extremely good looking revolution counter has been designed by Mr. Douglas Young, and we will let him describe it. Fig. 70. Some Meccano parts and, a tooth powder tin are required, and you needn't even solder unless you want to. The cost is around 1s. 6d. to 2s., with the tooth powder thrown in. It is dead accurate, counts up to 950, and has a zero setting for use at will.

The illustration shows the counter attached to the most popular winder in use today. But one screw, apart from the winder jaws, holds it, so it can be adapted to fit any winder. Get first the handsome and professional-looking (when it's painted) case, the carbolic tooth powder tin. This one costs threepence, and measures 3in. x 1 1/4in. It must not be any smaller. The other parts required are Meccano rods, two 6in, and two 3in. long, two worm gears, one 19-tooth x 1/4in. gear-wheel, and one 50-tooth gear-wheel and five collars.

Punch a hole in the dead centre of the top and the bottom of the tin. Do this with a spike which burrs the edges inwards; this makes a better bearing than a sharp-drilled hole. Bend a 3 in. rod at right-angles in the middle, file one of its ends into what you think a smart pointer should look like. The other leg of the right-angle will henceforth be referred to as the centre shaft (P) and must be fitted in the holes just made. Use the bottom of the tin with its raised edges as the "face," and the lid as the "back," which is left removable for inspection.

Fit the 50-tooth gear (G4) to the centre shaft, bush first. Next comes shaft B, a 3 in. rod revolving in bearings burred inwards at points which can be judged visually. This is not as difficult as it seems, since there is a great deal of leeway provided by the fact that G4 need not mesh with G2 dead on its axis. Thread on shaft B a collar C1, 19-tooth gear G3, worm gear G2, and collar C3.

Fig.70Fig.70

Now the winding hook. A 6in. rod has 3 1/2in. kept straight in the vice, while the rest is hammered into the hook. The worm G1 is threaded on, and again the tin case is punched

to make bearings, this time through the lid-flange. It leaves only 1/16in. of the tin outside of the bearings to hold the shaft, but it is enough. The lid will have to be nicked to fit over this, and a similar nick in the lid will have to be cut to fit over rod D, which fits parallel and similarly to rod A. Rod A, of course, revolves, held in position by collar C4, while rod D is fixed. Fixing can be done by soldering collar C2 to the tin and a dab of solder on the point where the rod goes through the tin. While the soldering-iron is hot, put a 5-hole Meccano strip behind the face, with a dab of solder through its outer holes, to reinforce the rather weak tin.

If you are determined not to get the soldering-iron out, leave out this strip and substitute a threaded collar for the plain collar C2, fixing it to the tin with a screw into its end. You will then probably have to use threaded rod in place of plain rod D. Keep rod A at right-angles to rod B, and, as a guide, the measurements between centres on mine are, rod A to centre shaft P, 9/16in., P to D, 3/8in. A piece of 1/2in. tin strip folded over rod D, drilled to fit under a screw conveniently situated on the winder, holds the counter still when it wants to turn, and finishes the mechanism.

The worm gears revolve once to turn the flat gears one tooth, therefore the total reduction is one to 19 x 50 = 950. Stick a paper disc on the "face." If you know how to divide it into 9 1/2 divisions, each representing 100, you can calibrate it accurately. If you don't, then start from 12 o'clock, wind 100, and make a mark, similarly all round.

The zero-resetting device is optional, but jolly well worth adding. It consists of spring "S." Don't get scared of making a spring on the score of not being a metallurgist. Heat treatment is not necessary. This was made by unwinding a Meccano spring to get a few inches of straight wire, then winding it round a piece of wire somewhat smaller than the rod, for about six or eight turns. It springs out to a size which grips the centre shaft tightly. When the lower end is secured beneath a screw and nut put through a hole in wheel G4, it allows the pointer to be turned by hand clockwise, but not anti-clockwise. The top end is left free, and collar C5 is to stop the spring riding up. Start the spring from the bottom, coming up clockwise.

You turn the pointer clockwise to zero by hand after each wind, yet, when winding, the pointer positively registers without slip or springiness.

Coloured aero dope finishes the metal, and put plenty of transparent dope on the paper face. Everybody will want to borrow it as soon as you bring it out, so put a clockwise arrow on the face to show which way to set it to zero.

Here are a few reminders to finish the chapter. Lubricate the motor well and truly, but . not so much that the lubricant flies all over the place. Don't over-wind the rubber. A revolution counter is a great asset.

 
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