Back in the June 2006 issue of Contractor we had a look at one of Caterpillar’s most successful motor scrapers of the 1950s, the model DW20. Since that article was published some more data has come to light and, given the luxury of having a few more pages these days, we can have a look at the DW20 in a little more detail.
There were six versions of the DW20 manufactured – the 6W series, specifically designed for use with the company’s W20 bottom dump wagon, the 21C, 57C, 67C, 87E and 88E, which could all be used with scrapers or whatever else the customer wanted.
We will not examine the 6W series in this feature as it was basically the same as the 21C series and was purposely built with a dedicated hydraulic system for bottom dumps only.
So, the first of the real scraper tractors was the 21C, originally powered by a Roots blown Caterpillar D337 6-cylinder diesel which was produced from 1951 through to 1955.
Equipped with the Caterpillar No 20 scraper rated at 14 cubic yards struck and 22 cubic yards heaped, this was the model that paved the way for the rest of the series and it underwent a few modifications along the way.
The original 225 horsepower Caterpillar D337 engine suffered from a few cracked cylinder heads due to excessive exhaust back pressure, so a slightly wider cylinder head was designed to alleviate the problem and this incorporated twin exhaust stacks to allow the engine to ‘breathe’ better. This was introduced into the production line as soon as the modified engines became available.
Having cured this problem, Caterpillar also changed the transmission from a 5-speed manual to incorporate an overdrive or auxiliary box function which boosted the machine’s top speed from 26 to 35mph and effectively gave the machine 10 forward gears.
From its inception, the DW20 was fitted with a Caterpillar No 27, 2-drum cable control to handle trailed scrapers. This was a “live-drive” unit powered by a driveshaft that came off the engines PTO. It was rugged and dependable and required infrequent adjustment. It had a line speed of 907 feet per minute and usually used half-inch cable.
By the mid 50s, the DW20 was regarded as a reliable and productive earthmoving tool so it was really no surprise that Caterpillar would want to develop it further.
In 1955 Caterpillar introduced the DW20E which incorporated several major improvements over the previous 21C series machine.
For the first time, customers were given a choice of starting methods – direct electric or the usual 2-cylinder petrol starting engine, which had been a Caterpillar standard since it introduced diesel engines to its machines in 1931.
The DW20E direct electric start version was known as the 57C series and the petrol starting engine version was called the 67C series.
Changes were not isolated to starting methods as Caterpillar had dispensed with the Roots blower, turbocharging the D337 engine which now produced 300 horsepower.
Tubeless tyres were used for the first time, 29.5x29s in place of the old narrow tread, tubed 24.00×29, and an all-new scraper, the No 456, rated at 18 cubic yards struck and 25 cubic yards heaped, became standard.
This proved to be a very popular combination for Caterpillar and it was produced with few changes up until 1958.
Next, and as it proved, last off the production line were the DW20F and the DW20G.
Known initially as the DW20F (87E series direct electric start and 88E series gasoline engine start), horsepower had been given a boost to 320 horsepower and an optional larger capacity scraper, the 24 cubic yard struck, 34 cubic yard heaped No 482 was offered. Use of this scraper required that the machine be tandem push loaded in order to fill the bowl in a reasonable amount of time.
A redesign of the radiator and air induction system produced two recognisable spotting features in the form of twin air cleaners and a radiator header tank that projected above the bonnet line.
For a short time, Caterpillar offered for the DW20 (and DW21) a form of power shift transmission known as “Synchro-Touch”.
This was an electro-pneumatic device whereby the operator dialed in the gear he wanted, depressed the clutch and the transmission would shift without him having to move any levers.
A little too ahead of its time, Synchro-Touch was subject to damage from dirt and condensation which could often result in more gears being selected than required resulting in the inevitable dire consequences for the gearbox.
All machines so equipped were retro-fitted with the standard 10-speed transmission at Caterpillar’s cost and Synchro-Touch was quietly forgotten about!
By yet again increasing the engine output, this time to 345 horsepower, the DW20G was created, the last of the line. Caterpillar chose not to change the serial number prefixes for this modification so the ‘F ‘became a ‘G’ at 87E508 and 88E1261 respectively.
As the DW20 by now had reached the apex of its development potential, and more modern machines were being offered by Caterpillar’s competitors, the DW20 was finally dropped from production in 1960 and replaced with the model 630A.
The New Zealand Connection
Big users of this type were W Stevenson & Son which operated a large fleet of DW20s of several versions at the Kopuku opencast coal mine in the Waikato where they proved themselves to be true stalwarts. As time passed and the fleet began to suffer from age-related engine problems, a group were re-powered with Caterpillar model 1693 truck engines, a conversion that proved to be a huge success. In this configuration, some machines had working careers that lasted into the mid-1980s. Not a bad record for an “ol’ timer”!
For the Diecast Model Collector
There are five models of the Caterpillar DW20 available and these are as follows in order of size:
1:160th (N) scale DW20E & No 456 scraper made by Matchbox models in the 1950s.
A very basic but accurate model that can be hard to find and is quite expensive in good condition. Long out of production.
1:87th (HO) scale DW20G by Roco. This is actually a 630A with a No 482 scraper but can be re-worked into a nice looking DW20 with a bit of care. Has a number of working features and is reasonably well detailed. First released in the late 1960s, is now long out of production and increasingly hard to find.
1:50th scale DW20E & No 456 scraper by EMD. Well detailed mixed media model from Eastern Europe and available through select dealers in the USA. You will be very happy if you can get one, but your wallet won’t. Horrendously expensive.
1:40th scale DW20 6W/21C series with W20 bottom dump by Revell. This is a rather crude plastic promotional model made by Revell for Caterpillar dealers when the DW20 was introduced. More curiosity value than a true scale model.
1:25th scale DW20E 67C series with No 456 scraper by ACMOC/First Gear. An absolutely wonderful, museum-class model which is so realistic you can just about hop in and drive off. Made in a very limited quantity, they don’t come much better than this, but they are expensive. Worth the investment if you’re into scrapers.
Brief Specifications – Caterpillar DW20E – 67C series
Engine: Caterpillar D337T, 6-cylinder, turbocharged inline diesel rated at 300 horsepower @ 1800 rpm.
Starting method: Caterpillar 2-cylinder petrol engine rated at 25 horsepower.
Transmission: Caterpillar 5-speed constant mesh with 5-speed auxiliary gearbox giving an effective 10 speeds forward and 2 reverse.
Clutch: Air boosted 16” double dry plate.
Top speed: 33 mph.
Steering: Worm and recirculating ball with hydraulic booster.
Brakes: Air operated shoe type on drive and trailed equipment axles, synchronised to brake trailer first to prevent jackknifing.
Tyres: Front – 14.00×24, 16-ply.
Drive – 29.5×29, 22-ply E3.
Scraper – 29.5×29, 22-ply E3.
Scraper: Caterpillar No 456.
Capacity: 18 cubic yards struck, 25 cubic yards heaped.
Length: 43’ 10”.
Width: 11’ 9”.
Height: 11’ 3½” (to top of apron sheave tower).
Operating Weight: 27 tons (empty), 53 tons (loaded).