CaterpillarClassic Construction Models

The Caterpillar 639D elevating scraper

As in comedy, they say that timing is everything. Caterpillar introduced the model 639D just when the bottom was beginning to fall out of the large earthmoving project market place. By Richard Campbell.

The Caterpillar 639D was designed specifically to take on Wabco in the large (30 cubic yard plus) twin-engined elevating scraper market, and to this end it did a good job.

Wabco had had this portion of the earthmoving business entirely to itself since the introduction of its BT333F in the late 1960s and had been continually developing and improving on it, first with the 333FT and then the 353FT.

To be quite frank, Wabco made very good big elevating scrapers.

Caterpillar, which at this point had only ever manufactured single-engined elevating scrapers, was determined to have a slice of this action and set about designing a twin powered elevating scraper of its own.

Trials were undertaken with several prototypes at Caterpillar’s Arizona proving grounds until a satisfactory configuration was agreed upon.

The 639D as the machine came to be known, was not developed from Cat’s 637D twin powered open bowl scraper with an elevator mechanism inserted as quite a few people think, but from the lesser known 633D single engined elevating scraper.

Officially introduced in 1979, the Caterpillar 639D was a 34 cubic yard elevating scraper with 700 horsepower at its disposal, nitrogen/hydraulic cushion hitch and a top speed of some 35 miles per hour.

In the tractor unit, power came from a 450 horsepower Caterpillar 3408T V8 engine with a 250 horsepower Caterpillar 3306T inline six-cylinder diesel in the rear.

Initial sales of the new machine were good but there were storm clouds on the horizon with a global downturn in large projects looming.

Sales began to slow dramatically as the work on big jobs and opencast mine stripping dried up.

This downturn not only affected Caterpillar but all the major players in the industry and was responsible in part for the demise of International-Harvester, Fiat-Allis, Wabco and Clark-Michigan.

Sales of big scrapers, open bowl or elevating, were just not there so the decision was made to pull production of the 639D in 1984 after just five and a half years.

This should not be regarded as a bad reflection on the 639D, which was a very capable machine, but on the industry that spawned it.

Gone also was the 639D’s major rival, the Wabco 353FT, the downturn hitting Wabco so badly that it exited the scraper market entirely and sold off its earthmoving divisions to Dresser Industries.

At least Caterpillar had its other core products such as track type tractors to keep it going.

Total production of the Caterpillar 639D was not very great with less than 300 manufactured.

A high proportion of these were exported to Australia where they worked well in the sandy soils of Western Australia.

The machine was right, Caterpillar just got its timing wrong!

The 639D described

Using the tractor unit of the 631D/633D/637D series, the 639D followed conventional Caterpillar design practices of the day and was powered by a 450 horsepower Caterpillar 3408T 65° V8 diesel attached to a Caterpillar 8-speed powershift transmission with a variable input torque converter for optimal elevator loading.

A manually applied diff lock was incorporated into the front differential to prevent wheel spin in soft underfoot conditions.

Air operated shoe brakes were fitted to all wheels.

Caterpillar chose the 37.25×35 tyre for the 639D with other options available if required.

Steering was by the standard Caterpillar method of two double acting cylinders attached to multiplier linkages. These were fitted quite high up on the gooseneck and allowed 90° turns in either direction.

The 639D’s bowl held 34 cubic yards and it, and sister 633D’s bowls were unique, differing from all other elevating scrapers by featuring a tilting floor which moved back in an arc to dump, with the front edge of the floor acting as a strike off blade to spread the load as it moved backwards.

Twenty elevator flights packed in the load and were driven by a single hydraulic motor with two forward and one reverse speed, mounted on the left side of the elevator frame.

As with other Cat scrapers, the centre edge could be extended to a two-inch drop centre, or be fitted with bolt-on teeth for more aggressive loading in tough soils.

Unlike smaller elevating scrapers, the Cat 639D and its competitor, the Wabco 353FT, could be used in rock cuts within reason, as the large size of the machine and its elevator components pulverised the incoming material.

The 631/633/637/639D-series Caterpillar scrapers had one of Cat’s first integrated ROPS structures and provided a significant upgrade over what Caterpillar had previously offered on its motor scrapers. It was designed with the operator in mind and was a well laid-out operating environment.


For the operator there was air conditioning and extra sound suppressing material for the cab. Almost all production 639Ds were fitted with a ROPS cab even though an open ROPS was considered standard equipment.

Scraper additions included fenders, the previously mentioned bolt-on cutting-edge teeth and a fast fuelling system supplied by either Wiggins or Buckeye.

The New Zealand connection

There is one Cat 639D operating in New Zealand, owned by Hatuma Lime Co in Waipukurau. This was imported used from Australia and is an exceptionally impressive machine to see working. Hatuma has long been an advocate of elevating scraper mining and its fleet also includes some other machines which are now regarded as rarities including NZ’s only International 444 and Michigan 110HTs.

Good on you Hatuma!

For the model collector

Bleak! No manufacturer, either mainstream or specialised, has cast a model of the Cat 639D in any scale.

The 639D would not be an easy model to scratch build owing to the design of the bowl and elevator assembly.

It is hoped that one of the more forward thinking diecast manufacturers such as Classic Construction Models (CCM) will produce a 639D in the future.

*In September 2019 CCM  released 1:48 scale versions of the 639D and 633 scrapers.

Brief Specifications Caterpillar 639D

Engine (front):             Caterpillar 3408T, turbocharged, aftercooled V8 diesel rated at 450 horsepower @ 2000 rpm

Engine (rear):               Caterpillar 3306T, turbocharged, aftercooled inline 6-cylinder diesel rated at 250 horsepower @ 2200 rpm

Transmission:             Caterpillar full powershift 8-speed transmission with variable torque converter and two loading ranges to optimize

transmission performance

Top speed:                  35mph

Brakes:                        Shoe type, full air operated

Tyres:                         37.25×35, 42-ply E3

Steering:                      Two cylinder with multiplier linkage. 90° either way.

Turning circle:             40′ 7″

Hitch:                          Caterpillar Cushion Hitch with oil/nitrogen accumulators

Capacity:                    34 cubic yards heaped

Operation:                   All hydraulic

Elevator:                      Two-speed with one reverse, 200/300 fpm

No. flights:                  20

Length:                        47′ 8″

Width:                         13′

Height:                                    14′ 4″

Operating weight:       62 tons empty, 99.5 tons loaded

Caterpillar's 639D scraper

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