Of all the motor scrapers that Caterpillar has produced through the years, perhaps the most popular in terms of worldwide unit sales is the model 631. By Richard Campbell.
Originally intended as a replacement for the aging DW21, the first 631, the 631A was introduced in 1960.
We took a detailed look at the 631A in the July 2011 issue of Contractor.
By the standards of the time, the 631A was really only a stopgap step in the evolution of the 631 as although it featured a powershift transmission and planetary final drives, all the bowl functions were still entirely cable operated.
That all changed with the release of the 631B in 1962.
The 631B described
Caterpillar launched the bulk of its new “600” series motor scraper line during the spring of 1962.
There were 11 new machines in total including the 631B.
Rated at 21 cubic yards struck and 30 cubic yards heaped, the 631B was powered by Caterpillar’s 360 horsepower model D343T 6-cylinder turbocharged engine mated to a 9-speed planetary Cat powershift transmission. Top speed was around 32mph.
It was completely hydraulically operated and weighed approximately 35 tons empty.
During 1965 Caterpillar responded to an industry request for a “beefed-up” version of the machine to handle tougher going.
This resulted in the 631B Special Application which had an extra two tons of steel added around the bowl area to resist abrasion by rocks and rough material.
Unfortunately, around this time, there were a number of catastrophic gooseneck failures reported and customers also complained of not being able to use the machine’s higher transmission speeds due to the rough ride the machine gave.
The end result was the introduction of Caterpillar’s ‘Cushion Hitch’ which provided a number of benefits.
Firstly, it reduced the amount of stress in the gooseneck area and secondly, it prevented the ‘loping’ effect common to 2-axle overhung scrapers, giving the operator an improved ride by reducing the amount of shock transmitted through the tractor frame.
Caterpillar launched the upgraded version of the 631B in 1967, initially with cushion hitch offered as an option.
It also redesigned the tractor’s fenders, squaring them off, and boosted engine horsepower to 400hp at the flywheel.
In this form the 631B remained in production until 1969 when it was replaced by the 631C. Caterpillar built over 4000 631Bs.
The 631C described
Still powered by the D343T engine, which was now up-rated to 415 flywheel horsepower, the 631C featured a few refinements over the B series machines but capacity remained the same at 21 cubic yards struck and 30 cubic yards heaped.
The tractor unit was a lot more ‘squared off’ than the 631B’s tractor unit to streamline the manufacturing process and a larger radiator was installed to cope with the higher horsepower of the engine.
An all-new 8-speed semi-automatic transmission was fitted allowing a useable top speed of approximately 34mph.
In this form the machine weighed around 39 tons.
While cushion hitch was initially still only offered as an option, almost all purchasers opted for this item and it was made standard equipment from serial number 67M5012.
Caterpillar manufactured over 5500 631Cs.
The 631D described
Commencing in 1975, an all-new version of the 631 became available, the 631D.
The 631D was a complete redesign from the ground up and was powered by the new 450 horsepower Caterpillar 3408TA V-8 diesel with an 8-speed semi-automatic Cat transmission.
While struck capacity remained at 21 cubic yards, heaped capacity rose to 31 cubic yards and empty weight went up to 46.5 tons.
An extremely popular machine, the 631D was a great success story for Caterpillar which sold them by the thousands both in the USA and abroad.
They were particularly popular in Europe as they could handle large yardage projects without being too large themselves.
As a measure of the 631D’s success, very few modifications were carried out during the machine’s 10 years in production although you can spot an older 631D by where the headlights are positioned.
The 631E described
Introduced in 1985, the 631E was a further refinement of the 631 series to carry the type into the 21st century.
Retaining the Cat 3408TA engine and its 450 flywheel horsepower rating, Caterpillar installed a new type of 7-speed semi-automatic transmission.
Bowl capacity remained the same.
The 631E featured several design changes to the machine’s panelwork and improved access for servicing.
Empty weight of the machine had now increased to almost 50 tons.
Like former versions of the 631, it was extremely popular amongst users and operators.
A revision of the 631E took place in 1991 producing the 631E series II.
Principal changes included an engine upgrade to the Cat 3408E TA which produced 490 flywheel horsepower and a return to the 8-speed semi-auto powershift transmission.
Although the machine had shed a little weight, reduced to 49 tons through some redesign, top speed had dropped to 33mph.
The 631E series II was manufactured from 1991 through to 2001 when it was replaced by the 631G, there being no ‘F’ model of the type.
The 631 remains in production today as one of Caterpillar’s stalwart types, the current version of which is just about to go into full production as the 631K.
During its lifespan, the 631 has seen its entire competition come and go and is now the only 21 cubic yard open bowl motor scraper offered by any manufacturer!
The New Zealand connection
A very popular motor scraper in this country, various 631 models have seen extensive service, especially on hydro projects.
New Zealand dealer Gough, Gough & Hamer has imported examples of all the 631 variants mentioned in this article although there are no longer any 631Bs left as they were sold and exported offshore around 1980.
Fleet users included Green & McCahill, Ross Reids, WH Butson, Earthmovers Waikato and W Stevenson to mention just a few.
For the diecast model collector
For once, the model collecting fraternity does have something to add to their collections.
Although there are no models of the 631B or 631C available in any scale, there are some fine models of the 631D and 631E.
Recently bankrupted Spanish model producer Joal produced a model of the 631D to 1:70 scale.
Apart from the odd scale and questionable tyre tread pattern, the model is very good indeed and represents a mid-1970s’ production machine. It has been released several times and can be acquired quite cheaply.
UK model producer Black Rat has also made the 631D, but to 1:50 scale.
The Black Rat model is one of the finest you can add to your collection, the only drawback being the price and the very limited numbers that were produced.
It was made in at least four different versions with only 50 of each produced in each run and has a working cushion hitch.
They are highly collectable but fetch over US$1500 at auction.
The 631E was manufactured in model form in 2013 to 1:48 scale by Classic Construction Models (CCM) in a limited run of 750 models.
This, like the Black Rat model, is a stunning example of the model maker’s art and is very accurate. Like the Black Rat model it also features a working cushion hitch.
NB: There is no specification breakdown as this is a general overview.