Tracked vehicle suspension design 3

[HT79]

Where can I find information on suspension systems for tracked vehicles (ie. heavy equipment, half-tracks)?
I am looking for everything from design theory to parts suppliers. Any resources either on the web or in print would be very helpful. Thanks

 

[harrisj]

I've been designing suspension systems for military tracked vehicles for about 30 years in UK. These are mostly high speed vehicles (in tracked vehicle terms - ie about 50 km/h plus) and up to 70 tonnes.

Lots of ways of doing it - leaf springs, coil, volute, torsion bar, hydro-pneumatic etc - just like other vehicles. The best way depends on what you want to do and how fast (and how much dosh you've got).

Main problems are achieving enough vertical wheel movement and eliminating slack track. Stopping it all falling apart is also pretty important.

Tell me what type of vehicle you are working on.

Regards - John

 

[rlkinze]

I'm also quite interested in suspension design. I am working on an off-road "personal tank" for my disabled daughter. Clearly, small and light are the main factors. Specific questions:
-Can torsion springs be used to distribute vehicle weight via idler wheels?
-Any methodology for establishing minimum belly height?
-Any drive tumbler design information for use with elastometric track?
Regards,
Robert

 

[GregLocock]

-Can torsion springs be used to distribute vehicle weight via idler wheels?

yes, this was a common design on full size tanks.

-Any methodology for establishing minimum belly height?

Use the beer-can rule! For non-Australians the 'legal' assessment of whether a car has been lowered excessively is done using a beer can. If you can drive over it without knocking it over then you are OK. Quite why the police drive around with beer cans in their cars is another matter... ie about 5 or 6 inches. This will handle almost any reasonable obstacle. If this thing is 3 ft by 3ft then you'd probably get away with 4 inches, and some common sense from the driver.

-Any drive tumbler design information for use with elastometric track?

If you can find a track manufacturer or supplier they wll advise you what sprockets to use, I should think.


Cheers

Greg Locock

 

[harrisj]

Robert:

As Greg says, you can use torsion bars on the road wheels (srictly speaking the idler is the return wheel at the other end of the vehicle to the drive sprocket - road wheels run along the track and support the vehicle's weight).

Suspension is a good idea, as it helps get weight down to the tracks evenly, as well as providing comfort etc. But if it's too soft, you may have problems with track coming off. I assume you're taliking quite low speeds, so damping shouldn't be a problem - probably enough friction in the system.

Belly height depends on the obstacles you are driving over. If you're looking for real cross-country capability, 200mm would be good for a small vehicle. If it's just around the garden, over kerbs, footpaths, 100 to 150 would be OK.

If you're in UK, try Thistle belting in Scotland. Run by Fred Powada who's an expert on belt tracks.

Don't underestimate power needed to turn by skid-steer, especially on soft ground. Remeber that short tracks spaced widely make it easier to steer and inhibit roll (good) but have less directional stability and tend to pitch (bad).

A good ratio is 1.2 (known as the L/C ratio - length of track on ground / width between track centres)

Hope this helps - John

 

[rlkinze]

John, Most helpful information, especially the general guidelines and for providing proper terminology.

This will, for several reasons, be an electric vehicle, resulting in a bit of weight for the batteries. The comment on skid-steering was well taken; I am struggling somewhat in trying to estimate the drive output power requirements. The target weight is approximately 200 kg/450 lbs at a maximum speed of 10 kph/6 mph. Are you aware of any general guidelines on power to weight ratios?
Regards,
Robert

 

[harrisj]

Robert:
Each track supports 100 kg. If you're on tarmac with rubber tracks, coeff of friction is 0.8 - 1.0. So you need 90 kg of tractive effort on each side.

But tracks are turning the vehicle as well as skidding. For a L/C of 1.2, add 40%. If L/C = 1, add 25%. If L/C = 2, add 80% (these are rules of thumb - I can't justify them mathematically - they seem to work!).

So with L/C = 1.2 you need enough sprocket torque to provide 126 kg of force on each side. So with 200mm dia drive sprockets, you need 126 x 9.81 x .1 = 124 Nm of torque. Quite a lot but you're not going fast, so lots of gearing.

Remember that dc motors can provide prodigious amounts of torque for short periods, like steering. But your control electronics will have to cope with the maximum.

Wheels are easier but not so much fun.

John

 

[rlkinze]

Very helpful information, John. Thanks. Skid-steering a vehicle with wheels would, unfortunately, result in far too much bouncing about for this particular application; my daughter's disability makes her about as strong as a half-drowned kitten, with a predisposition towards injuries if she is bounced about. So tracks are more practical (in addition to being more fun).

I suspect that the friction on non-paved surfaces should generally be lower that rubber on tramac, assuming that the track does not "sink" substantially into the road surface. Is this a reasonable assumption?

Also, how important is it for the drive tumbler to have deep engagement with the track? I'm looking into a sort of double-sided timing belt as a track, but am unsure as to if debris would cause track engagement problems.

Thanks,

Robert

 

[harrisj]

Robert:

Friction only approaches unity for dry rubber on concrete / tarmac. As you say, other surfaces (grass, dirt) are considerably less. Just keep away from areas with severe sinkage - it's too difficult for a light-powered machine.

As you have deduced, debris is a real problem. On sand or gravel, the spoil is carried on the track like a conveyor belt and interferes with the drive system. On an excavator or a tank with lots of power, it is simply squeezed out of the way. Fist sized rocks are crushed to powder by the steel tracks and the huge amounts of tractive force available. Skeleton track obviously helps as it provides an escape route for the debris. But this is a difficulty with rubber belt tracks and low power.

You have two requirements for the track drive system; one is to guide the track over sprocket, idler and road wheels (and return rollers if you have them) without rolling off the side. During a neutral turn (pivoting) there is of course a huge side force on the track. This is reacted on a tank by a spike or 'horn' that projects inwards from each track link, which engages with a groove in the rotating components. On a belt drive, you can use side plates on the rotating bits to keep the track on, but they do tend to get blocked with dirt, damaged by impact, etc.
The second requirement is to transmit torque; this can use the horn but usually relies on a hole at each end of each link which is driven by teeth on each edge of the sprocket.

Look at the Diehl (German) website to see some typical solutions.

Unless you have a large budget, you could find a suitable size industrial timing belt and ask a local rubber company to modify it for you with some sort of drive / location pegs on the inside surface. They would have to vulcanise these onto the belt - not sure how durable this would be. You might even consider fixing a series of metal or plastic (acetal?) cones on the inside using countersunk bolts from the outside.

If your drive components are timing belt wheels, you could machine relief grooves and/or holes in the running surface to allow debris to escape. Your belts will be sized for ground pressure considerations not torque transmission so you can rely on less than the full quota of drive tooth engagement.

John