David Brown Transmission Engineering

In the early days of David Brown, transmission work was a speciality, and it will be recalled that Meltham Mills produced tank transmissions during World War 2. The Merritt-Brown transmission was one of the most successful developments in tank production, and down to the present David Brown Textron still has a major role in the supply of gearboxes for military application. As a result, really good design and production teams became established at Meltham Mills during the early 1940s, and their influence continued down to the very end. The old faces may have been long gone by then, but the apprentices who had passed through these departments had gained a knowledge of the operation.

A select few took up the mantle and became the new experts within this sphere of the firm’s operations. Generally the transmissions produced by David Brown were first class, but that is not to say that there were no problems whatsoever. Generally speaking our gearbox was so successful that the one used in manual transmissions right up to the 94 series was still recognisable as being similar to that used in the Cropmaster. it was a unit assembled with the differential consisting of two cast-iron end plates carrying the bearings for the driveshaft, layshaft and pinion shaft, with the front and rear end plates being joined by four steel spacer bars.

However, as power transmission increased, a cast one-piece spacer replaced the spacer bars. The transmission was available as a sixor 12-speed unit. in the case of the six-speed box, the pinion shaft was fitted with three gears high, medium and low being driven by the layshaft that, in turn, was driven by either a high or low gear on the driveshaft giving six speeds. Reverse gear was mounted on a bush on the drive shaft and was in constant mesh with the layshaft. Reverse was engaged by sliding the low gear on the pinion shaft into mesh with the reverse gear on the drive shaft giving two reverse speeds.

In the case of the 12-speed box, the 12 speeds were attained by the addition of an extension to the front end plate carrying another layshaft known as the dumbbell layshaft. This was driven from an extended driveshaft, and it was either in direct drive giving the six speeds already mentioned above, or in ’reduction’ giving a further six speeds. There were four speeds in reverse.

When using the reduction facility of the dumbbell layshaft on the early models, it was not intended for heavy draught work. Improved lubrication of these gears and shafts on later models meant that it could be used as a full 12-speed box for all types of work, both heavy cultivation and light row crop work. It is worth noting that when the hydraulic system was in constant pumping, for example when operating a loader, the low-pressure lubrication system is inoperative. Therefore, it was imperative to return such oil to the transmission through the correct port in the rear axle case, in order to keep the lubrication system working. The six-speed box was controlled by two levers and the 12-speed box initially by three, and latterly by two levers, but in Q cab versions it reverted back to three levers.

In 1971 synchromesh was added to the box, giving on the move changes from first to second to third and back to second. Constant mesh high and medium gears now replaced the sliding high and medium gears on the pinion shaft and the drive from the layshaft was transmitted via dog clutches to the pinion shaft. A synchro was situated between the high and medium gears. The introduction of a synchro box meant that, when changing the transmission oil and filter, instead of finding small bits of chipped teeth from unsuccessful double de-clutch gear changes in the filter housing, one would find bits of disintegrated synchros!

Up to the 1490, all our tractors had a differential containing two bevel pinions, but the 14 series and six-cylinder models had four bevel pinions. Due to the bevel pinions on the 1490 picking up on the pin, bushes were fitted in the bevel pinions bores but the problem was found to be misalignment in machining the pin holes in the cage and the Inspection Department’s failure to find the problem. All tractors were provided with a differential lock, which operated by sliding on the right-hand spur pinion shaft and engaging with the differential cage, whilst disengagement was effected by a coil spring.

David Brown also designed and built a semi-automatic transmission to meet customer expectations, although on this point the late Tom Lazenby made the astute comment: ’What tractor buyers thought they wanted, and what they really wanted were two completely different things. Either you had a job to sell them a tractor they really needed to operate in a cost efficient or effective way, or you had to talk them down from something they didn’t need, but just wanted because a neighbour had something of the kind. This was especially true when it came to automatic transmissions, and we had the devil of a job persuading customers that this wasn’t always a good idea but if that is what they demanded we decided we’d make the best there was.’

There were three main competitors: Ford had their select-o-speed (nicknamed select-a-jerk); Massey Ferguson had multi power; and, IH had hydro. Browns produced the Hydra-Shift, which gave four-speed clutchless changes in three forward and one reverse range, and was by far the best of the quartet. To emphasise this, we might point out that it was in production with only nominal changes from 1971 to 1988. What is more, the Hydra-Shift transmission earned the company the Queen’s Award to Industry for Technological Achievement in 1974 and a Design Council Award in 1976.

The range box had three forward ranges designated Creep, Field, Road and Reverse. This was a constant mesh gearbox with a helical drive to the layshaft from the drive shaft, which was also utilised for the reverse range. Fixed and sliding dog clutches provided the drive to the pinion shaft. Change on the move was not a requirement in the range box as the semiauto part of the box overlapped in each range. It was easy to set off hauling a heavy load in Road 1 and change up the Hydra-Shift gears as you got rolling. The range box was a robust and reliable unit giving little if any trouble.

The four-speed part of the transmission consisted of two planetary reductions of different ratios, 1.37: 1 for the first and 1.82: 1 for the second reduction (later changed to 1.78:1). Each planetary reduction was equipped with a brake band, which was applied by a coil spring and released by hydraulic pressure. The same hydraulic pressure after passing through a sequence valve would clamp up a clutch pack via a piston in the centre of the reduction, thereby giving direct drive. Trevor Williams remembers spending many hours working on such things as the orifice size in the sequence valve and the rates of the cushion springs in the pistons to achieve acceptable change characteristics. The following table gives the ratio of engine speed to gearbox drive shaft speed entering the range box:

Hydra-Shift positionFirst ReductionSecond ReductionRatio
2Direct Drive1.82:11.82:1
31.37:1Direct Drive1.37:1
4Direct DriveDirect Drive1:1

Hydraulic pressure was supplied by a pump integral with the gearbox giving 68-72lb/in2 for early models and up to 87-91lb/in2 later. This oil supply was filtered only by a gauze filter and was susceptible to fine contaminants entering the system that could result in the relief valve sticking in the open position. When the oil was warm, this sometimes would prevent any gear being obtained other than first. The change times for a Hydra-Shift box should be in the order of one-second up changes and twoto three-second down changes with the oil at operating temperature and the engine speed at 1,800 rev/min. The transmission was not to be used as an automatic ie leave it in fourth position and set off as this would result in slipping of the clutches as the pump tried to lock up both reductions at once. Early models up to 90 series had a vertical control spool whilst later ones were fitted with a horizontal spool to give clearance for the cab floor.

Three tractors, one 1490 and two 1690Ts, were built with an experimental push button control on the Hydra-Shift. This comprised two solenoid valves replacing the spool valve and four buttons mounted on the dashboard (one for each gear). Everyone who drove it praised the system, but it was never built on production models.

Another occasional problem with the brake bands was that the locking nut and washer could work their way off and, although these could be refitted quite easily, it was a difficult job on tractors with saddle tanks. Yet, when looked after with clean oil and the required filter changes, it was a reliable gearbox.

Hughie Cartwright still owns an ex-field test six-cylinder tractor disguised as a 1412, which has now completed over 13,000 hours with no trouble. The transmission oil in a Hydra-Shift always ran hotter than in a synchro and an oil cooler was provided for the 1412 and 1690s.

As four-wheel drive became more popular, the demand for a 4WD Hydra-Shift grew. In fact Field Test were running a 1490 4WD prototype disguised as a 1210 with a Hydra-Shift in the late 1970s, and the front wheel drive gave no problems. However, the Hydra-Shift 4WD was not available until the release of the 94 series in 1983, and then there were problems with the drive to the transfer box. This was cast into the gearbox housing on production models as opposed to the fabrication used in the experimental tractors and this casting could break under extreme heavy loads. Field Test were asked to determine the causes of the failure and, using a 1494 loader tractor, we managed to break the casting on a particularly arduous loading operation that necessitated repeatedly reversing up a very steep incline with virtually all weight and drive on the front axle. A strengthened casting was introduced, and it proved satisfactory in service.

From the gearbox, drive was taken to the rear wheels via spur reductions and the pinions were crowned to concentrate the loadings in the middle of the tooth. Due to surface pitting of the teeth, the reductions on the higher-powered tractors were shot-peened to lengthen their life. However, we were running epicyclic reductions on the 145hp experimental tractor, as these proved to be more reliable for transmitting the higher torque loadings. The final drives could be swivelled down to give a low cost high-clearance tractor for use in taller crops. The brakes, either drums or oil immersed discs, worked on the spur pinion shaft. Speaking of brakes we should point out that David Brown had always used drum brakes up to 1974, but, with the introduction of the 14 series tractors, oil immersed discs were introduced on the larger tractors. Drums, however, continued to be used on the 1394 right up to the end of production. The size of the drums and shoes was increased over the years to cope with the larger loads that were hauled by these tractors, but it was found that these brakes could fade under continuous heavy use.

I remember, that after reports of this happening, we took out a 1390 tractor and had towed it round the roads with a constant pressure on the brake pedal to see if fade occurred. After some time I indicated for the driver of the towing tractor to pull in, but when he did I ran into the back of him because the 1390s brakes were totally inoperative by this time. Due to the heat that had built up there was no friction between the shoes and drums, but once cooled the brakes worked again.

The Girling oil immersed discs were excellent brakes, which consisted of six rotating bronze discs each side separated by stationary steel discs. These discs were applied by an actuator with six steel balls in the centre that ran up ramps, and, as the brakes were applied, this tended to give a self-servo effect in the forward direction. The only problem that I can recall with these discs was the pronounced ’clunk’ that could be heard as they wore when braking in forward then reverse ie loader -work. But fitting an additional stationary disc, when the amount of wear allowed, cured the problem.

Another problem was the squawk produced by using the wrong type of oil, but we found the best oil to be Shell Donax TT. We had an accelerated brake test on the test track, which consisted of loading the tractor to maximum weight and driving forward in H2 and reversing in HR and applying a standard pressure to the pedals at each change of direction. This test went on for days and brake fade never occurred. These types of brakes are still used by DaVid Brown Textron in some of their axle sub assemblies.

David Brown always had a good reputation for their Power Take Off units, which were bolted to the rear axle case and driven direct from the engine clutch via a carden shaft with a six-square spline output shaft or – on the six-cylinder tractor – 21 involute splines. We offered a multispeed unit, which gave 540 or 1000 rpm on a six-spline output shaft or a reversible shaft unit with either six splines (540rpm) or with 21 splines (1,000rpm) for high power applications. Hughie Cartwright recalls his first tractor fitted with a reversible shaft PTO, a late 1412, but said that the four bolts securing the shaft could work loose. However, after retightening once it would be okay. Alan Earnshaw remembers being given the job of purchasing a large supply of Loctite nut locking fluid which was put in small black bottles and then supplied free of charge to those customers who complained.

The PTO units fitted to lower-powered tractors had sliding spur gear engagement of the output shaft, whilst the units fitted to higher-powered tractors had constant mesh with dog clutch engagement. The reversible shaft PTO had constant mesh helical gear drives for reliability under long hours of use. The majority of Selectamatic tractors were supplied with a Livedrive two-stage clutch – ie one where drive to the tractor wheels could be interrupted but still allow the hydraulics and PTO to operate. Depressing the clutch pedal through a second stage disengaged the latter; however, it tended to be rather heavy on the PTO stage, due to the reduced mechanical advantage of the release levers on the second stage.

On the 1200 Browns introduced the hand-operated independent PTO clutch which was found to be easier to operate than the two-stage Livedrive clutch. Ken Jagger who spent many hours in Field Test working on clutches recalls that, during the early 1970s Browns, started to use a Laycock two-stage clutch. It was thought that if wear of the transmission and PTO clutch plates was similar, then internal adjustment was not necessary during the life of the clutch assembly. However, very little wear occurred with the PTO clutch plate and this then necessitated a complicated adjustment of the clutch through an inspection plate under the clutch housing.

Smaller PTO clutch plates were tried but the wear was still not equal and the problem persisted. On one occasion Laycock fitted some clutch facing material with solid rivets; however, the plates used to airlock to the flywheel and refuse to release.

When 12-speed boxes were introduced, it was found that there could be a problem with the transmission driven plate bursting. This tended to occur when the driver allowed the tractor to freewheel with his foot on the clutch whilst in a creeper gear, as this caused the plate to spin very fast. In turn this produced more centrifugal force than it was capable of standing whilst not clamped between the flywheel and pressure plate. Various materials were tried for friction linings including ceramic pads and sintered material, but the latter had a tendency to belleville making it impossible to disengage the clutch.

To observe how these performed a hole was cut in the side of a clutch housing and, with the use of a strobe light, the clutch could be viewed in the operating mode. In the field it was more difficult, but the problem still had to be examined. However, one dealer failed to solve the recurring problem of burnt out clutches on a 996-loader tractor, so it was immediately returned to the factory. The tractor was stripped down, carefully checked for alignment and, with nothing apparently amiss, it was deemed fit for active service.

The tractor was then loaned to another customer who put it to work on an arduous loading job with no problems. However, a second tractor that had been given to the first customer immediately began to experience further clutch problems, so Ken Jagger and Robert Booth, from the Service Department, visited the farm in question. It was discovered that the driver had a dislike of his David Brown and had wanted the farmer to purchase a John Deere, but when this had not happened he decided to abuse the DB by slipping the clutch, thereby overheating it and causing premature failure. Other problems of clutch slip and premature Wear could be caused by incorrect adjustment. To adjust the hydraulically operated clutch on a Q Cab tractor, for instance, the return spring was uncoupled and any free play between the lever and the slave cylinder eliminated. After this, you would back off a specified number of turns and tighten the lock nut, and this would allow clearance for the clutch release bearing.

However, some operators rather than backing off, would introduce more turns, thereby partially releasing the pressure on the clutch plate and making it easier to slip. A self-adjusting transmission clutch was introduced on the 1690T; however, there were some scary moments when we tried them in Field Test. What we had not realised Was that the Laycock release levers used to push out against the release bearing at higher speeds due to centrifugal force; therefore, the initial clutch pedal movement was used in taking up the resulting clearance rather than disengaging the drive to the transmission.













VAD6 50D


900 Series


VAD12 2D


950 Series


850 Series


880 Series


990 Series