The Caterpillar 633 scraper

Not wanting to be left behind in the market stakes for large single-engined elevating scrapers, sales of which had really blossomed during the 1960s, Caterpillar, who had already tested the waters with its 16½ cubic yard J619 elevating scraper, decided to go a little larger and compete head on with Wabco’s 333F and International Harvester’s E295. By Richard Campbell

The result was the model 633 which was basically the hard working and job-proven 631B tractor and a 32 cubic yard bowl which was jointly designed by Caterpillar and Johnson Manufacturing.

Following a short test period, the machine was put into series production in 1966.

Featuring an ejection system that is still unique among elevating scrapers, sales of the 633 were quite modest in comparison to its rivals.

When the 631C tractor made its debut in 1969, the 633 also received the new tractor unit, the combination then becoming known as the 633C.

Capacity remained the same at 32 cubic yards but horsepower was increased from 400 to 415 and the machine also had the new 8-speed Caterpillar semi-automatic powershift transmission that made the 633C not nearly as sluggish as its predecessor.

This was a far better machine than the original 633 and it cemented Caterpillar’s position as a viable contender in the 30 cubic yard elevating scraper market.

A further upgrade in 1975 produced the 633D.

Featuring a completely new, redesigned tractor unit, this was a real beast of a machine with ample power from its 450 horsepower Cat 3408T V8 and its success was such that Caterpillar developed it further by adding an extra engine in the rear to create the 639D (of which more in an upcoming article).

The 633D was the last of the 633s to use the tilting floor ejection mechanism.

Next and penultimate development of the 633 series was the 633E that appeared in 1985.

This featured an all new sliding floor ejector and even more horsepower, now up to 475 at the flywheel, plus a redesigned operator’s area with all the latest gadgets.

By now all of Cat’s competitors in this size scraper class had vanished but Cat undertook one more upgrade of the machine, producing the 633E series II before withdrawing the type from production around 1996 due to a worldwide slow down in the sales of large scrapers.

The 633 Described

We will look at the original 633, the 44J series that started the type off.

As mentioned earlier, the 633 used the tractor unit of the existing 631B motor scraper, with a few modifications.

Power was supplied by a 400 flywheel horsepower Caterpillar D343T, 6-cylinder, turbocharged and aftercooled diesel.

One of Cat’s great engines, the D343 featured twin overhead cams with automatic valve rotators and was used in a wide variety of other Cat machines as well as stationary and marine applications.

This was mated to a 9-speed powershift transmission (the 8-speed semi-auto came later) that gave the 633 an effective top speed of around 32 mph.

A higher capacity hydraulic pump was installed to cater for the greater hydraulic flow requirements of the elevator motor.

Brakes were full air operated S-cam activated shoes on all wheels with the scraper brakes sequenced to activate first to help prevent jackknifing.

Standard tyres were listed as 33.25×35 with other optional sizes available for special applications.

Steering was exactly the same as on all Cat 2-axle scrapers with the cylinders mounted high up on the gooseneck and working through multiplier linkages to achieve a 90° turn in each direction.

The original 633s were not fitted with Caterpillar’s cushion hitch and this item didn’t appear on 633s until the 633C was introduced in 1969.

A 13 flight hydraulically powered elevator fed the cavernous 32 yard bowl and could top it off in just on 60 seconds in good going. This featured four speeds including reverse to help in unloading.

Where the 633s bowl differed from all other elevating scrapers was its tilting floor which moved back in an arc to dump the load, the front edge of the floor acting as a strike off blade to spread the load.

This method of ejection was retained right up until the 633E was launched and quite why this particular method of ejection was chosen over the accepted industry standard for elevating scrapers of sliding floor is unknown.

A substantial spreader bar was set at the front of the bowl to which the ram ends of the two hoist cylinders were attached. Below this was the 3 x section cutting edge which could be equipped with five teeth if necessary for work in hard packed soil.

The operator sat on a partial suspension seat placed over the front left wheel with a full set of gauges on two panels bisected by the steering column.

Visibility was partially obscured to the right by the large aircleaner pre-cleaner but the view to the cutting edge was very good.

To the operator’s right were the bowl, elevator and ejector control levers, directly attached to the valve bank which gave good feel and pretty much instantaneous action.

Optional extras were few – cab, heater & retarder. Even the windscreen was an optional extra!

The New Zealand Connection

The 633 would appear to have been just too big or specialised for local contractors’ applications as the author can find no record of any being imported into New Zealand.

For that matter, none of its single engined competitors made it to our shores either

There is however, an example of the 633’s larger brother, the 639D, but the nearest 633s can only be found in Australia unless some enterprising soul imports one!

For the Diecast Model Collector

I’m afraid the pickings are really quite thin here as the only Cat 633 model the author is aware of is a 1:87th (HO) scale limited offering of a Caterpillar 633D by CCM (Classic Construction Models).

This is made of brass, is very expensive, and of course, being a limited production item, is quite hard to find as well.

If 1:87th scale is your thing it is worth tracking one down as it is beautifully made and very realistic.

*In September 2019 CCM released 1:48 scale Cat 633 and 639D scrapers onto the market.


Caterpillar 633 Brief Specifications

Engine:                        Caterpillar D343T 6-cylinder, turbocharged & aftercooled diesel rated at 400 flywheel horsepower @ 1900 rpm

Transmission:             Caterpillar 9-speed full powershift, barrel type planetary.

Brakes:                        Shoe, full air, S-cam operated

Tyres:                         33.25×35, 32-ply E3

Top Speed:                 32 mph

Steering:                      Full hydraulic, 90° each way.

Turning Circle:            40’

Capacity:                    32 cubic yards

Elevator:                      Hydraulic driven by vane type motor

No.Flights:                  13

Speed:                         4-speeds from 150 to 300 feet per minute

Length:                        43’ 10”

Width”                          12’ 6”

Height:                        13’

Operating Weight:       40 tons (empty), 76½ tons (loaded)

The Caterpillar 633 scraper

Related posts

Caterpillar’s Diesel Sixty: A landmark machine

Contrafed PUblishing

The Caterpillar DW15 scraper

Contrafed PUblishing

The Caterpillar 631B scraper

Contrafed PUblishing