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Slot.it. gears

How DO they do that??
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wixwacing
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Posts: 1871
Joined: Thu Jul 10, 2008 9:22 am

Slot.it. gears

Post by wixwacing »

One of the few remaining slotcar mysteries I have been avoiding is the mystery of the replacement spur gear and pinion. Why a mystery? Well, firstly, as a sidewinder gear it is essential that the pinion and spur are spaced apart at a concise distance which will ensure quiet running and longevity. Spur gears too far apart will age quickly for the simple reason that tooth contact will become reduced and harsher. One of the main requirements for good tooth wear on any gear is the necessity for more than one tooth to be in contact at one time, for as long as possible. Ideally, three teeth should be in contact at the same time. How’s that? Well! One tooth should be casting off (leaving mesh), one should be in full mesh (on a centre line drawn between both gear centres) and a third should be engaging. Three touching at once!



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On the minute engineering involved with slotcars it goes that just having two in mesh would be favourable and indeed, quite possible. One casting off and the other engaging. Another benefit of having more than one tooth in mesh at any one time is noise. Noise in gears can be caused by a poor or wide mesh. This can occur when a tooth has cast off before the next has engaged. This will cause the gear to ‘hop’ to the engaging tooth, generating noise and wear. All this is simple science and common knowledge to gear designers but to the humble slotcar enthusiast, unless he has a grounding in engineering, will seem obscure and pointless.

Where is this heading? I’ll tell you! Are you sitting comfortably?

Sidewinders as RTR slotcars are commonplace now and eventually, and especially for the competitor, the occasion will come that a gear will need to be replaced. Either because it has worn out or more commonly, because the drive ratio is less than suitable and needs to be changed. With a burgeoning collection of makes on the after market, it can be hit and miss trying to match gears to the model. The first criterion is the ratio required. This is easy enough to establish using charts and programs, or even a calculator to determine the starting point. Once an optimum ratio has been decided upon it is then necessary to match pinion and spur so that they achieve a good mesh using the shaft centres of the model. Too much backlash will see the gear wear rapidly and too tight a mesh will see the gears bind and the model operation become very noisy.

Optimum mesh should be almost zero backlash for brass bushed axles and models with ball bearings can be touching with just a tad of pre load. This is fine in models with infinitely adjustable motors like the 24th scales and wing cars and womps but RTR cars have fixed motor and axle mounts and gear mesh is going to be in the selection process and not a later adjustment.

Suffice it to say that if you are changing a spur gear you should really be replacing the pinion too! Even if there is no obvious problem with it and especially if the new spur is a different make to the equipment. If it is of any age, it will have worn in sympathy with the old spur and may well be too worn for a good starting mesh with the new spur. If the pinion is wide and the same make as the new spur, you may be able to position the gears so they mesh on fresh tooth area.

So, getting back to gear choice, we need to choose the correct gears. Using Slot.it products as an example the optimum ratio is already decided and how to achieve it will be the careful selection of spur and pinion. Slot.it have a choice of four pinions for sidewinders. 10z, 11z, 12z and 13 z. their spur gears come in a variety of teeth totals. The pinions, although varying in tooth count are all the one diameter, 6.5 mm. The spurs come in two diameters. 18.0 mm and 19.0 mm. This is a move towards getting the correct mesh for individual models. No doubt Slot.it have done their homework and have chosen these two diameters to enable their product to be used on most of the mainstream models as well as their own. Both Fly and Scalextric have a large range of models with sidewinders and anyone wanting to alter their final drive ratio will find these gears ideal. Just the ratio to decide on and Bob’s your uncle!

Other non mainstream models will require a little more research. The distance between the axle and motor shafts will need to be calculated and the appropriate spur gear selected. So how do we go about it without buying a whole bunch of gears and popping axles and motors endlessly in and out, changing pinions and spurs! Unfortunately, the ability to accurately measure dimensions using a Vernier gauge will be required. Not to mention a Vernier!



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Some dimensions need to be established. Firstly, a motor shaft is 2.0mm in diameter. A standard 3/32 axle is as good as 2.38 mm. To calculate the distance between shaft centres, the distance between outer shaft edges is measured. The shafts are added together and halved. This measurement is then subtracted from the total shaft spacing.

Ie.

Overall distance 13. 55 mm
Total shaft diameters (2.00 + 2.38) = 4.38
Deduct half of the shaft measurement (2.19) from overall distance:
13.55 – 2.19 = 11.36

11.36 mm is the distance between shaft centres of the model. The next job is to determine the most suitable gear / pinion combination. As all the pinions are the same diameter, it is a simple choice of spur gear.

The pinion is 6.5mm diameter and the first spur is 19.0 mm. This gives a total of 25.5 mm but because of tooth overlap, when the gears are meshed and the overall width is measured we get a reading of 24.3 mm! This tells me there is 1.2 mm overlap with these gears and this has to be taken into consideration when calculating shaft centres.

We know that the centre points of the pinion and the spur are going to be half of their individual measurement so; the centres distance can be calculated by firstly adding the two gear diameters.

6.5 + 19 = 25.5 mm
25.5 halved is 12.75mm. (this is centre to centre before tooth overlap correction).
Deduct tooth overlap : 12.75 – 1.2 mm = 11.55mm

11.55mm is the measurement from centre to centre for a Slot.it pinion and 19.0mm spur gear in mesh. This would give an interference fit if applied to the 11.36 example above, so now the 18.0 mm spur calculation needs to follow:

6.5 + 18.0 = 24.5 mm
Halved is 12.25 mm
Deduct overlap , 12.25 – 1.2 = 11.05

11.05 mm centres would fit the above example of 11.36 mm and have 0.31 mm reduced overlap which would give a loose mesh.



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Lastly, as mentioned in the opening stages, choice of pinion needs to be a part of the calculation from another point. A 10z pinion will suffer from minimal tooth contact and although it may be the only option in some cases I would avoid its use if necessary. The 11 and 12 z pinions are more user friendly as they have a good two tooth contact when turning. The 13z is a great idea and high in tooth contact but once again, because of the fixed diameter of the gear and the fixed spacing of the shafts, the teeth are in close proximity. The pinion would need to be backed off a bit to minimise the peaks on the spur wearing prematurely through too much side tooth contact with the pinion. (The engaging spur tooth runs a real risk of gouging the following pinion tooth if meshed too closely).

So there it is in a nutshell, as they say. If you didn’t follow the logic then don’t worry too much, someone at your club or a friendly hobby shop staff member or even another member on another board may have the solution for your particular sidewinder gear dilemma! This started off as me thinking aloud after being posed the question in the main forum and has got totally out of control!!
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When I'm not racing slotcars,
I'm out in the back yard, burning food!!

When I win, it's because of my talent, not my car or my controller!
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