By Fred Powada, Operations Manager, Thistle Polymer Composites.
Times change and people have almost forgotten how during the Provisional IRA bombing campaign in the UK, civilian casualties rapidly became “unacceptable” even for those sympathetic to the Provisionals, and the focus switched to targeting the military and specifically EOD operatives, who were “military” personnel. As a countermeasure, stand-off capabilities were developed using whatever was at hand, with wheelbarrow the classic image and description. Borrowing from armoured vehicle design, tracks were seen as the norm for all terrain capability, and the items at hand were Overhead Camshaft Drive (OHC) belts with modified wheelbarrow wheels for idlers and
traction. Not any old OHC belt as they were and are generally small and narrow with small drive teeth at about 10mm pitch, all for high rotational speed and high installation tension, also easily clogged and prone to “tooth-jump”. At hand were those from a Chieftain tank engine – really large OHC belts 2.1metres long and 75mm wide with drive teeth of 22mm (XH) pitch. Suspect devices (often “dummies”) were put in hard-to-get-at places, “warnings” given and the “trap” set. It was cat and mouse stuff coming to involve stair, kerb or rail traversing “issues” as the terrorist’s aim was that the EOD operative had to discard the stand-off capability and become vulnerable once again. Different types and designs of lugs were added to make the tracks better able to engage obstacles and provide traction.
Early tracks, to obtain purchase, were with the belts outside in and so turned into a flat belt drive with “stapled-on” lugs at 44 or 66 pitch. The Wheelbarrow as it had then become known moved for a time to a central Vee style of belt friction drive. The width was 76mm, lugs 49mm pitch and (a critical factor, especially for climbing) a fabric “facing” in contact with the ground, inevitably meaning that “grip” was less than ideal. Perfection was not achieved (it never is) but it worked!
But the EOD community did not and does not generally recognise that there is a parallel civilian application for stair climbing which is a completely different “world” and different criteria apply. In the main, tracked wheelchair, pallet, safe and photocopier “barrows” have to go up stairs. Slippage cannot be tolerated (as well as being potentially lethal to the operator/passenger, a friction drive consumes energy). These vehicles in the main have “stuck” with toothed “rubber” tracks. Some of these modern 96 tooth XH “rubber” tracks would fit early wheelbarrow prototypes, and give excellent performance. Now saying the tracks are rubber is a complete misnomer as they are a carefully designed composite construction as in Figure 1.
Compared to a friction or a chain drive, these can be relatively lightweight, intrinsically quiet and efficient. A “toothed” driven track works most efficiently with LOW track tension and this puts less strain on the structure of the vehicle.
For the track manufacturer, customer feedback on stair and obstacle climbing, while vital, can be confusing. Separating “vehicle” and “track” contributions to propulsion & climbing efficiency is tricky. There are no rules set in stone, and in any case many of the conventions relate to “normal” industrial belt drives; for instance, suggested minimum pulley sizes. A track which suits one vehicle does not necessarily suit another AND there is also the multiplicity of stairs in existence to consider – the answer is to break the problem down into “parts” and have, on a test vehicle:
• a platform with track length variability
• the possibility of varying load geometry
• the operator in control so he or she can “feel” what is happening.
Figure 2 shows a typical stair and they are not as varied as first appears. They comply with ISO standards and are designed to suit humans. Some stair coverings are an issue but it is generally the steepest stairs which are the problem. A stair “rise” of more than 220mm is rare so the diagonal is round about 300mm. And this means that for the classic “barrow” optimised for stair climbing, a lug pitch of 76 – 79mm is used and a vehicle with a pair of 50mm wide tracks carries 350kg up stairs. The 79mm pitch lug “feels” better going up stairs and the 76 going down. Most prefer the barrows to be optimised for use going down. They have to work on stairs up to 45° (rear fire escapes!)
A barrow track length needs to be from 2 to 2.2metres (to bridge three stair “noses”) but there are a whole range of designs in use.
For decent performance, the Wheelbarrow, which for a time had used steel conveyor chain as tracks, with moulded on rubber lugs, returned to a composite rubber track of patented design from Thistle Polymer Composites. The 76-pitch lug was the start point as optimum stair climbing performance was a requirement; a novel, 38mm pitch drive tooth was available from Thistle with a fabric facing for low friction and durability. The tooth facing fabric, tooth rubber, aramid cord tensile reinforcement, core rubber, lug reinforcing fabric and lug facing rubber were each optimised for function. The smooth and vibration-free drive characteristics of the new track brought favourable comment. Sprocket material, track guidance and track support have been modified, but the track design has stood the test of time and is still in use today (see Figure 3).
A lug pitch of 66mm (which is three XH teeth of about 22mm pitch) looked “sensible” and 82mm might also seem okay but neither are good
“climbers” – the track “jumps” on the stairs especially when going down stairs. No rule is absolute. Tall thin lugs (what we have come to call “Centipede” tracks again developed by Thistle Polymer Composites) give an inbuilt suspension. This combined with flippers to increase chassis length, suddenly make a 66mm lug pitch track an ideal proposition to give a vehicle platform with good “agility” shown, here on the NIC First Responder.
Tracks can have disadvantages as the heavier a track is, the proportionately more energy it consumes. Also billions of hard cash has been spent developing pneumatic tyres for automotive and off road use and for flattish ground wheels are hard to beat.
But there is the problem of stair climbing. For the “Cutlass” program, Northrop Grumman’s development engineers were sure something extra was needed for climbing and that pneumatic tyres would never meet specified requirements. Knowing the detailed background work carried out on tracks by Thistle, they asked that it be applied to tyres. The constructions and designs tried were from solid through double and single celled with 27 differing lug designs trialled. The work would cover several papers in its own right. There were vital inputs from the vehicle and track design teams and an extensive test program. The result, involving both inspiration and perspiration, is a “single-celled” design with a lug, called the “C” form, already tried and proven for a Wheelbarrow track, set at about 76mm pitch; this gives a vehicle with the efficiency of tyres, (the elasticity of rubber being exploited), which yet has climbing capability. After the event it all sounds straightforward, but it was anything but! To show the principles involved, here, (and not normally seen), are rear views of a “Cutlass” tyre.
The lug was known to be optimised for traction, the cell form provides compliance but interestingly has to be substantially undeformed when in contact with the stair nose. The construction, which looks straightforward, is in fact carefully crafted with different sections carrying out different functions with differing material. All to give just the right properties and achieve the best that is possible.
To help see how the “Cutlass” tyres “perform”, as initially the mechanisms involved were a puzzle, a static test facility was built by Thistle Polymer Composites to examine at both room and any elevated operational temperature just how the tyre/cell/lug interacted with the ground and stairs. This also allows the collection of load/deflection data to then predict the performance of alternative options and sizes and is a step closer to the real world than 3D computer modelling.
There is a wealth of possibilities for celled tyres. ■
ABOUT THE AUTHOR
Fred Powada, Operations Manager at Thistle Polymer Composites, has been making Rubber Tracks and Special Belting for vehicular use and the UGV community since the early days of Wheelbarrow starting in 1978 with XH Timing Belt variants to date. Being based close to Aberdeen, the Oil Capital of the UK & Europe, Thistle is also involved in the provision of rubber/glass reinforced plastics downhole sensors for resistivity measurement. This mix of skills, known to the NGC UGV Design Team, proved ideal to develop the rubber “celled” tyre for “Cutlass.”
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