In production for just three years, the HD-20 was the stepping stone between the powerful HD-19 and the very successful HD-21. RICHARD CAMPBELL delves into its history.
Following on from its success with the model HD-19 (see Contractor, August 2009) Allis-Chalmers updated the machine in 1951. The result was the HD-20.
This was done for a very legitimate reason as in 1947 International Harvester had brought out its model TD-24 track type tractor which had knocked the Allis-Chalmers HD-19 from the “world’s most powerful track type tractor” slot. The horsepower race was on!
Allis-Chalmers, who at the time did not manufacture its own diesel engines, chose a General Motors Detroit Diesel model 6-110 diesel to power the new HD-20 (or HD-20H, as it is sometimes referred to).
A relatively new engine in the GM Detroit Diesel stable, the 6-110 was a big, powerful 660 cubic inch, six-cylinder diesel and was also the prime power unit of the new Euclid S-18 and TS-24 motor scrapers just coming into service.
A feature of the HD-20, as had been the case with the former HD-19, was the use of a torque converter in the transmission, a concept pioneered by Allis-Chalmers in earthmoving equipment as far back as 1946 when they first began to appear on HD-14C tractors.
A great majority of the HD-20s manufactured were used primarily as push tractors or tow units for scrapers due to their (for the time) high horsepower, weight and the lugging characteristics of the torque converter drive.
The Tractomotive Company, who built front-end loader units for Allis-Chalmers, manufactured a four cubic yard loader for the HD-20 and in this configuration the machine was known as an HD-20G. It is thought that very few of these units were actually built.
With the acquisition of the Buda Engine Company by Allis-Chalmers in 1953, the company was free to pursue and develop its own engines.
Allis-Chalmers manufactured approximately 3000 HD-20s before the type was discontinued in late 1954 to be replaced by the all-new HD-21, a tractor that was to have a long and very successful career.
The HD-20 described
As mentioned earlier, power for the HD-20 came from a General Motors model 6-110 diesel. A naturally aspirated, Rootes blown six-cylinder inline engine, in this application it produced 178 flywheel horsepower and had ample reserve capacity.
It was connected to a two-forward, one-reverse speed manual transmission by a torque converter that used diesel fuel as operating fluid. These early forms of torque converter were known as ‘diesel charge converter’ types and had pretty much disappeared by 1960 as technology advanced.
Although the HD-19 and HD-20 were of very similar appearance, one easy way of telling which was which was by the location of the engines manifold type muffler – left side on the HD-19 and right side on an HD-20.
Double reduction final drives transmitted power to the tracks and steering was controlled by the usual multiple disc clutches and contracting band brakes.
An 84-inch gauge machine, the HD-20 usually ran on 37-section tracks with either 26 or 28 inch wide shoes.
As was the case with most Allis-Chalmers track type tractors the operator was provided with a nice open deck and a comfortable, deeply padded seat.
Visibility all around was pretty good and included a tapered engine hood and angled fuel tank.
Allis-Chalmers were an early supporter of ergonomic design and all operating controls were placed within easy reach of the operator. Engine and transmission instruments were set on a vertical panel just to the right of the steering lever console and were very comprehensive for the time.
Maintenance requirements were also taken into consideration by Allis-Chalmers and most regularly serviced components were located with easy service access in mind.
Optional attachments
Aside from the already mentioned Tractomotive front-end loader conversion, both Baker and Garwood, long time suppliers to Allis-Chalmers, built attachments to suit the HD-20.
Most of Baker’s were hydraulically operated, and Garwood’s operated by cable.
Straight dozers, angle dozers and semi-U blades along with bush and rock rakes could all be fitted depending on customer preference.
Allis-Chalmers itself provided a “hard nose” push plate directly attached to the chassis for scraper pushing and a lot of units were configured in this way.
Towed attachments were, of course, numerous and could include scrapers and rippers from any of the major manufacturers of the time
As a rule, most units were supplied with a Garwood 241 rear double drum cable control unit although with a conversion plate and the appropriate splined coupling, a Caterpillar No. 25 rear cable control could also be fitted, several New Zealand based machines were set up in this manner.
From the operator’s seat
I got to operate an HD-20 in the twilight of its career, at Feast Contractors Dry Creek Quarry in the Western Hutt, one Saturday afternoon in 1972.
Equipped with a straight cable blade the machine was definitely tired but the engine started first time and ran smoothly.
Quite a brute of a machine, I wasn’t particularly impressed by the handling qualities although this probably had a great deal to do with the age of the machine. Next time I saw her was in the ‘graveyard’, stripped of her blade and ready for the cutting torch.
The New Zealand connection
It is uncertain just how many Allis-Chalmers HD-20s were imported into New Zealand as no real records exist anymore. I know of at least eight, and there may of course have been others.
Known users of the type include NZ Roadmakers, Dryden Construction, F&J Bognuda, AF Porter, Horowhenua Earthworks and Feast Contractors (which was the ex-Bognuda machine). Do any still survive?
For the diecast model collector
Surprisingly, for a machine with such a short production life, there is a good model available of the Allis-Chalmers HD-20.
It is to 1:50th scale and is a ‘limited run’ model made from resin, brass and white metal by a company called EMD models in Eastern Europe.
Realistic metal tracks are a feature as is some pretty fine detail for a 1:50th scale model.
It is available in four versions – bare with cab, with frame mounted push block, with hydraulic S-blade, and with cable S-blade.
Overall it looks to be a little short in profile but is a convincing replica.
A not inexpensive model, I know of only two outlets in the USA where it can be obtained: DHS Diecasts and Buffalo Road Imports.
Brief specifications – Allis-Chalmers HD20
Engine: General Motors 6-110, 6-cylinder, inline, Roots blown diesel rated at 178 horsepower at 1700 rpm
Transmission: Manual, two speed forward, one reverse with torque converter
Clutch: 16½” manual, over centre
Top Speed: 7 mph
Steering: Hydraulically boosted multiple disc clutches and contracting band brakes
Track Gauge: 84”
Rollers: 6 per side
Track: 37 section
Std.Track Shoe: 26” or 28” single grouser
Length: 16’ (without attachments)
Width: 9’ (without attachments)
Height: 11’ 2” (without attachments)
Operating Weight: Approx. 22 tons (without attachments)