Image: From a Caterpillar promotional spec sheet dated 1979, the businesslike lines of the 639D elevating scraper are very apparent. To the best of my knowledge, all 639Ds were manufactured with the ROPS cab. PHOTO: AUTHOR’S COLLECTION
They say that timing is everything. Caterpillar introduced its model 639D elevating scraper just when the bottom was beginning to fall out of the large elevating scraper marketplace, and projects were becoming smaller due to a worldwide downturn in earthmoving and mining, writes Richard Campbell.
The Caterpillar 639D was designed to take on WABCO’s dominance in the large (30-cubic-yard-plus) twin-engine elevating scraper market, and to this end, they did a good job.
WABCO had had this portion of the earthmoving business entirely to themselves since the introduction of their BT333F in the late 1960s, and had been continually developing and improving on it, first with their model 333FT, and latterly their model 353FT. To be quite frank, WABCO made very good big elevating scrapers.
Caterpillar, which at this point had only ever manufactured single-engine elevating scrapers, was determined to have a slice of this market and set about designing a twin-engine elevating scraper of its own.
Trials were undertaken with several prototypes at Caterpillar’s Arizona proving grounds, using an amalgamation of the company’s existing 633D elevating scraper and a 637D twin-engine conventional scraper as a basis until a satisfactory configuration was agreed upon. The resulting machine was known as the model 639D.
Officially introduced in 1979, the Caterpillar 639D was a 34-cubic-yard elevating scraper with 700 horsepower at its disposal, a 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 639D 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 earthmoving industry and was responsible in no small 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 decision should not be regarded as a bad reflection on the 639D, which was a very capable machine, but on the conditions at the time.
Also gone was the 639D’s major rival, the WABCO 353FT, the earthmoving downturn hitting that company so badly that they exited the scraper market entirely and sold off their earthmoving divisions to Dresser Industries.
Fortunately for Caterpillar, they at least had their other core products (such as track-type tractors) to keep them going.
Total production of the Caterpillar 639D was not very great, with less than 300 being manufactured.
Quite a few of these machines were exported to Australia where they worked well in the sandy soils of Western Australia. Unfortunately, the machine was right, but the times were wrong.
The nuts and bolts
As mentioned previously, the 639D used the tractor unit of the 631D/633D/637D series and followed conventional Caterpillar design practices of the day.
The front engine was a 450-flywheel horsepower Caterpillar 3408T, a 65-degree V8 diesel which was attached to a Caterpillar eight-speed semi-automatic 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.
The rear powerplant was a six-cylinder model 3306T, rated at 250-flywheel horsepower. Air-operated shoe brakes were fitted to all wheels.
Caterpillar chose the 37.25-inch by 35-inch tyre for the 639D, with other options available if required.
Steering followed 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 degree turns in either direction.
The 639D was fitted with Caterpillar’s parallelogram linkage cushion hitch to help iron out haul rod shocks. This was usually locked out when the machine was loading.
The scraper’s bowl held 34 cubic yards. It and sister 633D were somewhat 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 cutting edge could be extended to a two-inch drop centre or be fitted with five bolt-on teeth for more aggressive loading in tough soils.
Unlike smaller elevating scrapers, the Cat 639D and its chief competitor, the WABCO 353FT, could be used in rock cuts within reason, as the large size of the machine and its elevator components pulverized the incoming material for easier loading.
Standard and optional equipment
Featuring one of Caterpillar’s first integrated roll-over protective structures (ROPS), the machine provided a significant upgrade over what the company 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. Nearly all production 639Ds were fitted with a ROPS cab even though at the time, an open one was considered standard equipment.
Emergency braking was a standard item in case of an air system failure.
Scraper additions included fenders, the previously mentioned bolt-on cutting edge teeth and a fast-fuelling system supplied by either Wiggins or Buckeye.
For the model collector
You are in luck if you have deep pockets! Manufacturer Classic Construction Models released a 1:50 model of the Caterpillar 639D in 2020. The only problem was, like the real thing, very few were manufactured. In the case of this model, only 300 were issued. The model is excellent (one of my favourites in my Retro Contractors fleet), but now very hard to find and expensive due to the small number manufactured.
Fixing the system