The Caterpillar 631A scraper

A direct descendent of the 1950’s DW21, the 631A was introduced in 1960 and was the second of Caterpillar‘s new ‘600’ series motor scrapers to appear. By Richard Campbell

Having put in much research and development to provide a worthy successor to the DW21 series of motor scrapers, which had been very popular, the 631A, while carrying a similar payload, was physically larger than its predecessor and considerably more refined.

One of the big selling features over the previous DW21 was the addition of Caterpillar’s newly developed powershift transmission and planetary final drives in place of the former’s manual transmission and bull gear and pinion final drives.

A more simplified steering system was also introduced which dispensed with the former DW21’s somewhat complicated four-cylinder system in favor of a two-cylinder arrangement. Operation of the scraper, however, was still by cable.

Caterpillar was in the process of developing hydraulic scrapers for its product range but at the time the 631A was introduced it was still in the prototype stage.

One could possibly consider the 631A as an interim type, however, during the two years it was in production, the machine sold quite well.

Usually mated with the No.631 scraper rated at 21 cubic yards struck and 28 cubic yards heaped, the 631A could also be equipped with an Athey PR631 rock body rear dump or PW631 bottom dump

Manufactured for a period of two years at Caterpillar’s (then) recently opened Decatur, Illinois, facility, the 51F series 631A was replaced by the all-hydraulic 631B in 1962.

Limited production of the 631A scraper bowl assembly was also undertaken at the Caterpillar owned ex-Birtley works in the UK.

The 631A described

As well as developing new machines in the late 50s, Caterpillar had also been hard at work on new engine designs.

What was to become one of Caterpillar’s most respected engines, the model D343 was chosen for the 631A.

The D343 was a six-cylinder, 893 cubic inch displacement, inline, turbocharged diesel with after cooler and featured a double overhead camshaft design.

Putting out 335 horsepower in the 631A, the D343 was also used in the 769 off highway truck, 988 wheel loader, 824 wheel dozer, 983 track type loader, No.16 motor grader and 825 compactor.

Caterpillar’s newly developed barrel type planetary powershift transmission was fitted as standard, doing away with the manual transmission which had been a feature of all the DW21s.

This transmission featured nine forward speeds in three separately selected ranges – torque divider, direct and overdrive – with upshifting and downshifting being automatic within each range.

An optional hydraulic retarder could be specified at additional cost.

The planetary final drives featured a fully floating axle and mounted inboard from these were the brakes which were full air, shoe type, S-cam operated.

One of the features that Caterpillar promoted was the fact that the brakes were sequenced to operate scraper first, which helped to reduce the possibility of machine jackknifing, a very real danger during high-speed operation.

Standard tyres were 29.5 x 35 E3 type with a 28 ply rating.

As mentioned previously, the steering system differed from earlier Caterpillar overhung scrapers in that it utilised two cylinders in place of the previous four.

These were mounted quite high on the gooseneck and acted through a torque multiplying linkage allowing 90 degree turns in either direction effortlessly.

Mounted on the back of the tractor’s main case was the cable control winch (PCU), a double drum unit driven live from the tractors power take off. This PCU was a modification of the No.29 PCU in common use on D8H and D9E tractors of the period and was very reliable. It featured air-assisted controls to reduce the amount of pull required to activate it.

The bowl was of typical Caterpillar cable-operated scraper design and was based on the No.463 towed scraper with suitable modifications.

Half inch cable was used on all functions except the apron, which utilised three quarter inch cable. Approximately 800 feet of cable was required to rev the machine for operation.

Layout of the operator’s station followed usual Caterpillar practice, with the operator seated above and just forward of the left front wheel on a bucket type seat. Not a great deal of suspension was offered by the seat except for a torsion spring and rubber snubber device.

The instrument panel was bisected by the steering column, with the engine instrumentation and tachometer on the left panel and the transmission upshift indicator and other instruments on the right.

To the operator’s right was the transmission shift control lever and the bowl operating levers.

Engine heat could be diverted to the operator’s compartment in cold weather by use of a sliding panel, while there were several air vents which allowed cool air in during summer.

Visibility was very good all round and a credit to the designers at Caterpillar. The only real obstructions were the exhaust pipe and twin air cleaner stacks.

An optional full cab (manufactured by Crenlo) could also be fitted at customer request.

From the operator’s seat

I got to spend a Saturday operating a Cat 631A during 1976 while helping out a contractor who was short of scraper operators.

One particular memory was the smooth way the transmission shifted between ranges compared to other scrapers I had operated up to that point.

Although the beast was a little on the tired side by this stage in its career, it was nonetheless a satisfying experience and an easy machine to operate.

The New Zealand connection

Seven Caterpillar 631A machines were imported by New Zealand Caterpillar dealer Gough, Gough & Hamer and almost all were delivered to South Island customers.

Users included Owers Bros (who got the first two delivered), Roadways, W.Butson and Groundworks.

These machines did sterling service on the many hydro electric projects during the 1960s but seem to have disappeared after that.

I would be interested to know if any still exist in one piece.

For the diecast model collector

No models exist of the Caterpillar 631A in any scale. There is however a 1:87th scale model of a late production DW21 manufactured by Roco that could be converted at a pinch by an experienced modeller.

Otherwise, until the ACMOC commission a model or Classic Construction Models (CCM) get really creative, nothing is available.

Brief Specifications – Caterpillar 631A

Engine:                      Caterpillar D343TA, 6-cylinder, turbocharged and aftercooled diesel rated at 335 flywheel horsepower at 1900 rpm

Transmission:         Caterpillar nine-speed full powershift transmission

Brakes:                      Air operated S-cam, shoe type

Std.Tyres:                  29.5×35, 28-ply E3

Top Speed:               32 mph

Steering:                    Full hydraulic, twin cylinder, 90° each side

Turning Circle:         36’

Capacity:                   21 cubic yards struck, 28 cubic yards heaped

Operation:                 All cable

Length:                      42’ 3”

Width:                         11’ 9”

Height:                       11’ 8”

Operating Weight:   33½ tons (empty), 65 tons (loaded)


The Caterpillar 631a scraper

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