A big tractor in its day, The Allis-Chalmers HD-20 was another part of the power struggle that took place between Allis-Chalmers and International-Harvester during the 1950s. By RICHARD CAMPBELL
In previous issues we have looked at the track type tractor size and power race that took part between Allis-Chalmers and International Harvester.
Not a company to stand idly by, Allis-Chalmers counter punched I-H in 1951 when they introduced the subject of this article, the HD-20.
Based largely on the chassis of the previous model HD-19, the HD-20 featured the largest General Motors diesel ever fitted to an Allis-Chalmers track type tractor, the six-cylinder GM 6-110.
Putting out a very healthy 175 flywheel horsepower, only five horsepower less than the International TD-24, the Allis-Chalmers HD-20 weighed some 22 tons bare and so, once again, Allis-Chalmers had the worlds largest track type tractor!
Allis-Chalmers’ relationship with General Motors went back as far as 1938 when Allis-Chalmers, who at that stage did not manufacture any high horsepower diesel engines of their own, entered into an agreement with General Motors to supply them with engines.
This agreement lasted for 16 years until Allis-Chalmers bought up the Buda engine company and began producing their own powerplants.
This was in fact a good move by Allis-Chalmers as by 1954, GM’s Euclid division were beginning to introduce their own track type tractors which posed a direct threat to Allis-Chalmers’ market share.
Allis-Chalmers manufactured 3099 HD-20’s during a production run that lasted from 1951 through to 1954.
HD-20s were used on some significant projects in the USA including the St.Lawrence Seaway and the Garrison Dam in North Dakota (which used a huge fleet of principally Allis-Chalmers equipment including some custom made “Siamese twins”).
The HD-20 was replaced by the new model HD-21 – an all Allis-Chalmers manufactured machine – in 1955.
Nuts & Bolts – The Chassis
The HD-20s chassis “tub” was fabricated from high strength steel into which the powertrain components were inserted.
This gave them plenty of protection and also resisted frame twisting in service.
The machine’s back end was a very strong fabricated box which contained the steering clutches, brakes and bevel gear drive.
Final drives were double reduction and featured Timken tapered roller bearings throughout.
Multiple disc steering clutches were used and the contracting band brakes were hydraulically boosted to ease operator effort.
Easy maintenance was a hallmark of the HD-20 and the entire steering clutch/brake band assembly for either track could be removed from the tractors main case as a unit by removing the fuel tank and one cover plate.
As with the previous model HD-19, the HD-20 featured a torque converter drive transmission.
Allis-Chalmers had been a pioneer in the application of this device to track type tractors.
The transmission was a three-speed type with an auxiliary high and low range gearbox giving the HD-20 a effective four speeds forward and two reverse.
Top speed in second high range was 7 miles per hour.
Eighty four inch gauge, semi-‘H’ section box track frames carried six bottom rollers and two carrier rollers per side.
An eight-pack multi-leaf equalizer spring took care of track vertical oscillation while the track frames themselves were supported by a shaft through the end of the tractors main case.
On the former HD-19 this had been a weak spot causing track misalignment but the problem had been addressed in the HD-20 which was very rugged in this area.
Track shoes were 24-inch, heavy duty, single grouser with track chain tension being maintained by a hydraulically adjusted bell crank arrangement and recoil spring.
The HD-20 had a very clean deck and was easy to get on and off of.
Instruments were grouped in a small panel just to the right of the console which held the two steering clutch levers and throttle control.
Transmission shift levers were centrally placed emerging from the floorboards with the master clutch on the operators left.
The only other controls on the deck were the two foot brake pedals.
A deep and comfortable seat cushion and back was provided for the operator.
Attachments & Options
Up until the early 1960s Allis-Chalmers did not manufacture any of their own attachments.
Baker and Garwood were the major suppliers of both cable and hydraulically operated blades
Cable controls (PCU’s) were supplied by Garwood, Heil and Buckeye.
The vast majority of winches fitted to HD-20’s engaged in logging were supplied by Carco, although the occasional Hyster unit could be seen.
Optional attachments offered by Allis-Chalmers themselves included a cab with heater, engine hood, side screens and front bumper (mainly for logging applications), belly guards, track roller guards, and a chassis-mounted scraper push plate which was applied to a lot of machines.
As a point of interest, Allis-Chalmers supplied several HD-20s to their subcontractor Tracto-Motive for conversion into “HD-20G” track type loaders.
These were a huge loader for the day and, because of the very small number that were manufactured, are also very rare.
Spotting an HD-20
Identification of an HD-20 in old photos is relatively easy, much easier than spotting an HD-15 or HD-19.
On an HD-20, the dual oil bath air cleaners are on the left and the exhaust is on the right (as seen from the operators seat).
Also, the HD-20 has a very large manifold mounted muffler on the right side to dampen some of that sweet-sounding GM 6-110!
The New Zealand Connection
There were quite a lot of HD-20s imported into New Zealand during the 1950s by MZ dealer Cable-Price.
Regrettably all records of exact numbers were lost during one of the great clean-up purges which companies undertake from time to time and all the valuable history was ‘binned’
Known fleet users of the type included Dryden Construction and NZ Roadmakers.
Any readers with information concerning other contractors owning the type (particularly Wellington based units) are invited to reply to the Author via this website.
For the Model Collector
Well Yee-Har as they say.
If you want an Allis-Chalmers HD-20 there is a delightful 1:50 scale model available from EMD Models which can be acquired through Buffalo Road Imports, USA.
The EMD model is manufactured in at least three different versions – cable blade, hydraulic blade and bare for towing scrapers.
While not inexpensive, it is of good quality and worth adding to any 1:50 collection.
Brief Specifications – Allis-Chalmers HD-20
Engine: General Motors 6-110 6-cylinder, inline, Rootes blown diesel rated at 175 flywheel horsepower at 1700 rpm
Transmission: Two speed manual gearbox with supplementary high & low ranges
Clutch: Single plate clutch with a diesel charge torque converter
Steering: 32 disc per side steering clutches
Brakes: Contracting band brakes, hydraulically boosted
Top Speed: 7 miles per hour
Track Rollers: 6 bottom rollers and 2 carrier rollers per side
Std.Track Shoe: 24” single grouser
Length (bare): 15’ 11”
Width (bare): 9’ 2”
Height (bare): 11’ (to top of exhaust stack)
Operating Weight: 22 tons (bare)