GEARBOX CONSTRUCTION E.M.G.S. MANUAL ( 9) The Girder Gearbox

Transcription

1 GEARBOX CONSTRUCTION SHEET 1 The author, having been disappointed with a few of the commercially available gearboxes, decided to design and construct something to his own design. The Girder Gearbox Application The gearbox is an open frame construction intended to house a simple worm and worm wheel to enable a small electric motor transfer drive to an axle mounted at 90 degrees to the motor shaft and to fit on a driving axle freely between 4mm model locomotive frames. Except for a micrometer or vernier gauge, the construction requires neither special tools nor skills. The important ingredients are a little care, simple method and technique. General Description The principle components of the gearbox are four plates, two side plates holding the driving wheel axle and worm wheel and two end plates, one forming a motor or back plate, holding the motor and its spindle and worm and the other a front plate. The side plates are assembled first and separated initially with spacers. The end plates are assembled loosely onto side plates to allow accurate meshing of gears before final assembly by solder. When the working waste and spacers have been removed, the resulting construction looks a little like a box girder. The general shape of the finished gearbox is a little like this: Materials Side and end plates Spacers Matched worm and worm wheel Motor Screws Nuts 10 thou brass is ideal. Brass tube, the inside diameter sufficient to provide a loose but not sloppy fit for 10BA screws. The spacers will be recovered after construction. Grub screw fixing is essential Of your choice 8 x 10BA at least 1 inch long. Within reasonable limits the longer the better. The type of head is unimportant. The screws will be recovered after construction. 8 x 10BA. The nuts will be recovered after construction. Ramin knot free rectangular section semihard wood, available in lengths usually of about 6 feet, in standard sizes ranging from about 20 or so to 50 or so mm by 5mm thick. It is cheap and available readily from builder merchants. Builders and DIYers use it for beading and finishing. Only a few inches are required. A drill stand is helpful such as those used with an Expo drill. However it is not essential. With care a hand drill can be used. The precise shape and dimensions is left to you.

2 SHEET 2 Determine the size of the gearbox. The size of the gearbox will be determined by the minimum space needed to house a matched worm and worm wheel. The worm and worm wheel should be carefully inspected to determine these measurements. Determine the distance between the side plates When a grub screw is used to lock the gears onto the shafts, generally the worm wheel will have shoulders on either side of the wheel and the width of the wheel will often exceed the diameter of the worm. Determine the maximum width of the worm and worm wheel. A micrometer is important. When the maximum width of the worm and worm wheel have been determined add 10 thou for a working clearance. This will determine the length of the spacers to hold and separate the side plates during construction. If the thickness of the worm wheel is less than the diameter of the worm, shims (washers) will be required to pack the worm wheel centrally on the axle. Determine the width of the side plates The width of the side plates must accommodate whichever is the greater of both the length of the worm and the diameter of the worm wheel. Use a micrometer to accurately determine this measurement. Pay particular attention to which way round the worm will fit. Some motor spindles are a little short and may not give you a choice. Determine the height of the finished side plates - the height of the finished gearbox. The height of the side plates must accommodate the diameter of both the worm and worm wheel. Although not as critical as the other two dimensions measure this carefully. Ensure the finished dimensions of the gearbox will fit into your model. If necessary make up a small block of wood of the finished size and offer it to the drawing of your model or use it between the frames if they have already been constructed. 1. Manufacture of Spacers. Material - Brass tube. Prepare a piece of ramin about four or five inches long, the thickness of which is exactly equal to the length of the required spacers. If necessary increase the thickness of the ramin by gluing card or paper onto the wood to achieve the required dimension. Use a micrometer. Accuracy is important. Drill one hole vertically through the thickness of the ramin. This is where a drill stand may be useful. The size of the hole should give a clearance fit for the tube used to make the spacers. Cut four pieces of brass tube a little longer than the finished length of the spacer. The exact size in unimportant. Square one end. Place the ramin on a secure flat working surface. Place the squared end of the rough-cut brass tube into the hole just drilled. Ensure the bottom of the brass tube rests firmly on the working surface below the ramin. File the excess of the brass tube down to the surface of the wood. Remove the brass tube. Repeat this operation three more times. Within an acceptable tolerance, all four tubes will be of the correct length. 2. Manufacture the Side Plates Prepare two pieces of 10 thou brass at least twice as long as the finished side plates. Sweat the two pieces together so they form a single piece of 20 thou brass. File one long edge flat. A piece of plate glass used as a surface plate (or something similar) is essential. Mark the width of the side plate and carefully file to that marked line. Take your time and use a micrometer. Very carefully mark a centre line. On the centre line mark and centre pop two holes, one for the driving axle, the other for an inspection hole to determine the meshing of gears upon assembly. The distance between these two holes is a little less than the radius of the worm wheel. Mark and centre pop four holes near each corner for the spacers. Drill the holes and separate the two side plates and clean up. Using the spacers, assemble the side plates with 10 BA screws and nuts. Test the free running of the axle and the lateral free play of the worm wheel. The axle should run freely with less than 10 thou side play. Ensure the worm fits loosely between the side plates. If there is any tightness that can not be adjusted out, remake the sub-assembly with slightly longer spacers. When satisfied set to one side. 3. Manufacture of end plates One end plate will form the back or motor plate and the other will form the front plate. Take two pieces of 10 thou brass. The width should be at least twice the length of the side plate spacers and about one half inch longer than the side plates just set aside. Within tolerable limits dimensions are not critical. Sweat together and file one long edge flat. Lay the end plates next to the side plate assembly in the approximate locations they will have upon final assemble. (See diagram on next page). Mark a line across the end plate to show the approximate location of the centre line of the axle hole in the side plates. Within tolerable limits this can be approximate. From this line will be determined the approximate position of the hole for the motor spindle. Any small error will be adjusted when meshing the gears upon final assembly. On the centre line of the back plate mark another small line. Its position from the axle hole line is equal to the radius of the worm wheel plus the radius of the worm when both gears are meshed (distance A). See diagrams on the next and following pages. Drill a hole in the end plate equal to the diameter of the motor spindle or if it protrudes the spindle bush together with other fixing holes required for your motor. Drill four holes in the corners of the end plates 10BA clear. Sufficient clearance is required between the 10BA holding screws to allow the side plate assembly to fit easily between the end plates. The screws will be used to clamp the end plates together with a motor and worm onto the side plate assembly. This is necessary for the meshing process described later. Separate the end plates and clean.

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4 SHEET 4 4. Assembly Assemble the motor onto one of the end plates. Check that the motor is properly secured and functions freely. If the centres of holes for the motor fixing screws are misaligned it may not be possible to mount the motor or the motor may not function properly. Assemble the End Plates onto the Side Plates. Place four screws through the corner holes in the end plate fixed to the motor. Fit the worm to the motor spindle. Place the other end plate loosely onto the screws and loosely secure with nuts. If not already still in place fit a driving axle and worm wheel into the side plate assembly. Remove the top spacers on the side plate assembly and from the bottom of the end plates pass the side plate assembly upwards between the four screws holding the end plate assembly until the worm and worm wheel mesh roughly. Replace the top spacers in the side plates. Tighten the end plate nuts with finger pressure only.

5 SHEET 5 5. Meshing of Gears. The point of mesh between the worm and worm wheel can be seen through the inspection hole. Initially, adjust the mesh by sight. It will require careful manipulation of the end plates in relation the side plates and the position of the worm on the motor spindle. Connect the motor and run carefully. If the motor refuses to run or does not run smoothly carefully and lightly tap the end plates or side plates until running improves. It may also be necessary to reposition the worm on the motor spindle. Repeat these operations until you are satisfied with the function of the gearbox. Take your time. This activity is tedious and needs to be right. Do not rush this activity. If you can not obtain satisfactory running at this stage scrap the gearbox and have another go. Without disturbing the mesh secure tightly all screws and nuts. With all parts in place, including the motor, solder carefully the outside of the end plates to the outside of the side plates to form a fillet on the outside of the gearbox between the surfaces of two adjoining plates. Care must be exercised not to allow too much heat to reach the motor. Ensure that none of the screws, nuts and spacers is soldered into position. The solder fillet need only be applied along those parts of the end and side plates where the finished gearbox will finally emerge. (This operation can be possibly made easier by slipping a small piece of tissue paper between the gears to establish a working clearance) 6. Finishing the Gearbox. I hope that you can now recognise that the extra length and width of the four plates used during construction is necessary to carry the holding screws and spacers needed to hold the plates but allow adjustment during meshing. When the gears are meshed the frame is soldered and the holding screws and spacers are then no longer necessary and can be cut away. When deciding how much extra material is needed during the construction consider carefully the position of the screws and spacers. Make sure the spacers separating the side plates do not foul the screws holding the end plates and they are both sufficiently far away from the finished gearbox so that when the waste is removed unwanted holes are not left behind. width of the side plates. Try not to make the front plate too small. If the front plate is made too small again the gearbox frame is weakened. At this point once any part of a solder fillet is broken the gearbox is lost. Having removed working waste there is then no way to remedy damage caused after that point. 7. Final Adjustment Assemble the parts back into the finished gearbox and test. The only adjustment now is the use of shims between the end of the motor and the back of the motor plate. Shims of very thin metal like aluminium foil set either above or below the centre line of the motor spindle will alter slightly the angle of the motor spindle passing through the motor plate and marginally alter the mesh of the worm and worm wheel. Only fine adjustment is now possible. If the meshing process described was not done adequately this adjustment may not compensate for excessive error during the meshing process. Again test the finished gearbox and make any fine adjustments to the mesh of the gears. Only when satisfied secure all grub and other locking screws. The gearbox and motor is now ready for your model. 8. Conclusion What results is a compact hardy open frame free fitting gearbox together with its motor providing opportunity to remove and refit any of the parts for which it was made. Although the prospect of making a gearbox of this type might appear daunting to some, writing this article has taken far longer than making the gearbox. It sounds more complex than it really is. Making the gearbox requires almost no special tools or skills and with care is not difficult. I hope you found this article interesting and that I have not made too many errors or confused you too much. My hope is it has given you enough ideas and interest and sufficient confidence to have a go. I made and scrapped four gearboxes before reaching the final design. My finished gearbox works well and gives me a lot of satisfaction. It will be going into a scratch built 4F. John Russell (Shropshire) Remove all screws, nuts and spacers readily accessible. Remove the driving axle and worm wheel. The worm and motor can not be removed until the top waste part of the front plate has been cut away to allow the worm to slide off the motor spindle and access is gained to the motor fixing screws. To prevent metal dust entering the motor place a small plastic bag over the motor and secure with Sellotape before cutting the top half of the front plate from the gearbox frame. It is helpful if most of the top part of the front plate does not have a solder fillet. With the top part of the front plate removed, remove the worm and motor. What remains is a girder like open gearbox frame plus lots of working waste. Remove the waste largely to your own taste. However you should not reduce the width of the back and front plates to the same width as the spaced side plates. This will destroy the external solder fillet and seriously weaken the frame. Try to leave at least 1mm of back and front plate protruding beyond either side of the