Wikipedia

H-drive

AnH-drivedrivetrainis a system used for heavy off-road vehicles with6×6or8×8drive to supply power to each wheel station.

Plan view of a DutchDAF YA-1264×4 truck.
The centre spare wheels are unpowered, but can roll freely.

H-drives do not use axles but rather individual wheel stations, usually carried on apunt chassis.A singledifferentialsplits the drive into separate left and right drive shafts, which each run fore and aft inside the bottom corners of the chassis. At each wheel station abevelbox drives the half shaft out to the wheel.

Advantages

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  • Each wheel station hasindependent suspension.
  • Greaterground clearancethan conventional axles, as there is no central housing for the differential andfinal drive.
  • Lowerunsprung weightas only the halfshafts are carried, not the centre differential of an axle.
  • Only one differential is required; however, this also requires that final drive to transmit all of the vehicle's torque.
  • Losing traction with one wheel still retains traction for the other wheels on that side; in effect, a longitudinaldifferential lockis permanently engaged. This does have the downside ofdriveline windup.

H-drive is not commonly used for 4 wheel vehicles, as it is relatively complicated for small vehicles. It has been used most widely for military 6×6 chassis in the West. Vehicles of theWarsaw Pact,such as theTatra 813andMAZ-535series, were instead based on narrowbackbone chassiswith a centralpropeller shaft.

DAF

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Post-warDAF YA-126[nl]

H-drive was first developed byHub van Doorneof the Dutch truck makerDAF.It was a derivative of theirTradoconversion to produce a6×4off-road truck from a commercial4×2chassis. The Trado used a bogie rear suspension for both sets of rear wheels. This suspension, best known through theScammell Pioneerof 1927, uses a single central axle, or driveshaft, that in turn drives twowalking beams(balanceur,in Dutch) one on each side. The wheels are supported by overhung stub axles. The conversion added the walking beams to the ends of the original truck beam axle. From that point, the drive between the axles of each side was separated side by side.

In 1938, a later version of the Trado 3 conversion added drive to the front wheels and so converted a6×4vehicle to6×6drive. Unlike most all-wheel-drive vehicles, the front axle was no longer a live beam axle with added articulation for steering, but used two separate drive shafts, one to each front hub.

This principle of divided drive already being established for the front of the Trado, it was a minor step for van Doorne to divide the drive to the rear wheels as well. TheDAF YA-328used walking beams where the axle was no more than a pivot and the drive was supplied entirely by external longitudinal drive shafts, one on each side.[1]As was usual for heavy vehicles of this period, thefinal drivesand right-angle drive to the stub axles were combined through aworm gearbox. This also had he advantage that it is easy to connect such boxes in series, using the rear end of the worm shaft as an output.

Stub axle worm box and driveshafts ofYA-328"Dikke Daf"

This type of drive was used by DAF for several types of military vehicles:

DAF also made cars and were particularly known for theirVariomaticcontinuously variable transmission,introduced with theDAF 600in 1958. Although at a different scale to their military vehicles, these also used the unusual principle (outside DAF) of a side-by-side divided drive.

Daimler scout cars

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Daimler Armoured Car

The first major production of the H-drive and the greatest numbers produced were for the BritishDaimler Armoured CarandDaimler Dingoscout cars of WWII.

As relatively small four-wheeled vehicles, these used a simplified layout of the H-drive. A single wide casing housed the differential andtransfer box,with four articulated driveshafts running tobevel gearboxes inboard of each wheel.[2]The use of bevel boxes, rather than DAF's worm gears, required the final drive reduction to be placed in the hubs, using anepicyclic reductionin each hub. This had the advantage of reducing torque in the driveshafts, allowing their unsprung weight to be made lighter.

Ferret scout car,sectioned. The transfer case is paintedeau de nil(pale blue) and one driveshaft may be seen running forwards.

In later years a similar layout would be used for theFerret scout car.This had an even more compact layout, with the gearbox and transfer case within a single housing.[3]The driveshafts were articulated withTractajoints and epicyclic reduction gearsin the hubs.[4]

An advantage of the H-drive was the low overall body height as the hull could sit between the suspension units, rather than above axles. This was demonstrated by the CanadianLynx Scout Car,a derivative of the Daimler Dingo but using conventional Ford axles, which was a foot higher overall (70 in, 1.8 m vs 59 in, 1.5 m).[5][6]

Alvis FV600 series

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Alvis Stalwart

H-drive is probably best known today through the Alvis FV600 chassis, theAlvis Saladinarmoured car, theStalwartand family. The initial requirement was developed by theDepartment of Tank Design(DTD) immediately post-war and the six-wheel, all-driven configuration with all-round independent suspension chosen on the basis of experience with the best of WWII vehicles from four to eight wheels.[7]The Saladin was designed as a 10-ton vehicle built on a welded steel punt chassis, forming an armouredmonocoquehull. It was to use the equally new 8 cylinderRolls-Royce B series engine,the B80.[8]The contract for development of the FV600 chassis was awarded toAlvisin October 1947.[9]By 1950, events of theMalayan Emergencyhad overtaken the British Army and with an urgent need for their firstarmoured personnel carrier,protected against guerilla ambush, theFV603 Saracentook priority over the Saladin.[10]

Saracen used an almost identical chassis and drivetrain to Saladin, although the engine was relocated from the rear to the front of the vehicle. The transmission used a 5-speedWilsonpreselector gearboxwith afluid flywheel.Reverse gear was provided within thetransfer box,allowing all five gears in either direction. The centre bevel boxes were included within the transfer box housing and had a slightoverdriveto the drive shafts fore and aft. Each wheel station useddouble wishbonesandtorsion barsfor suspension with four (three on the centre stations)shock absorbers.[11]Steering was applied to the centre wheels and a lever arrangement moved the front wheels by a proportionately larger amount.[12]

A mark of the FV600's chassis' success was its application across a range of vehicles of varying weights and operational requirements, with great success at each and with little need for variation between them. One difference was in the braking system; the Saracen useddrum brakes,the Saladinring brakes.In time, the heavier Stalwart would require more effectivedisc brakes.[13]

Driveline windup

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Alvis Saladin, with white-marked hubs to show windup

A drawback to the H-drive is the risk ofdriveline windup.When used for equally spaced wheels (i.e., rather than cargo trucks with close-set rear axles) the front two wheels are arranged so that both steer, the rear less so than the leading wheel. The varying track radii mean that when the vehicle drives in a curve on firm tarmac each wheel travels a different distance. As there is no differential action between the wheels on each side, this causes a lot of wind-up in the bevels and shafts.[14]

Standard operating instructions for the Stalwart recommend that after travelling some miles on firm ground, the vehicle should be bounced over a kerb or railway sleeper to lift wheels clear of the ground, one-by-one, so allowing them to spring back and release the windup.[15]Excess windup could easily lead to a broken gear in the bevel or hub gearboxes. To indicate this, white lines were painted across the ends of the hubs. Normally the lines should be parallel but as windup occurred they would become misaligned, indicating the need to release this.

See also

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References

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  1. ^E. W. van Popta; Cornelis Maria Schulten (1978).Nederlandse militaire voertuigen.Militair memoriaal. Vol. 3. Holkema & Warendorf.ISBN9026945434.
  2. ^Munro, Bill (2002). "A New Generation of Fighting Vehicles".Alvis Saracen Family.Crowood Press. pp. 24–26.ISBN1-86126-537-9.
  3. ^User Handbook for Scout Car Liaison, Ferret, Mk 1(PDF).TheWar Office.1957. p. 79.
  4. ^Daimler Ferret Scout Car Precis, Mk 1 & 2(PDF).TheWar Office.1957. pp. 32–34.
  5. ^"Daimler Dingo compared to a Canadian Lynx".14 September 2013. Archived fromthe originalon 2016-03-04.
  6. ^"The Ford Lynx Scout car".www.mapleleafup.net.RetrievedJun 7,2020.
  7. ^Munro (2002),pp. 23–24.
  8. ^Munro (2002),p. 30.
  9. ^Munro (2002),p. 34.
  10. ^Munro (2002),pp. 36–38.
  11. ^Munro (2002),pp. 40–41.
  12. ^Munro (2002),pp. 33.
  13. ^Munro (2002),pp. 118–120.
  14. ^Ware, Pat (1994).In National Service.Warehouse Publications. p. 118.ISBN0-9525563-0-8.
  15. ^Ware (1994),pp. 129–130.
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