Monday, 8 July 2019

Canadian Dry Pin

This was originally written as a submission to a question on r/AskHistorians. Reddit user Jeddaven asked "What disadvantages did Canadian Dry pin tracks face versus tradition tank tracks?" Since this is a topic with little detail available, I decided to reformat my answer and post it here.

Canadian tank production is one underappreciated aspect of the country's contribution to WWII. Montreal Locomotive Works made excellent use of the American M3/M4 chassis, building the Ram and Grizzly tanks, Sexton SPG using their chassis, and the Skink AA tank on the Grizzly chassis.

Initially, these Canadian-built AFVs used WE210 tracks, commonly used on Medium Tanks M3, which were then replaced by T54E1 tracks, also used on American tanks. A shortage of rubber made the Canadians explore an alternative. Fully metallic Canadian Dry Pin tracks were designed.

Canadian Dry Pin track link.

According to Canadian trials, CDP tracks could be expected to last for 1200 miles in severe conditions and 2000 miles in favourable conditions on tanks (Ram and Grizzly) or 1500-2500 on the lighter Sexton SPG. British results were much poorer, but that was because the track was installed backwards during trials. Trials were held again with the track the right way. I don't have details on these second trials, but later documents comment that were much more successful. Tracks would start stretching much earlier (for instance, one Sexton was stopped after 1252 miles), but it was possible to remove a handful of links and carry on. Other trials also showed that ~1200 miles was a more realistic lifespan estimate. For instance, after 1154 miles of running on a Sexton (479 on road, 675 cross-country) an inspection showed that the track pins were in good condition, but the links themselves were heavily worn, both the contact surfaces and the track pin eyes. A number had developed cracks. 8 track links have been removed up to that point. Canadian tests with Ram tanks showed that the lifespan of the tracks was comparable to that of American style tracks, and unusual wear on the drive sprocket was not observed.

Let's look at some data on American-built tracks to compare. The British kept a pretty thorough track of their tanks throughout the war, an AFV Technical Summary was distributed monthly, which included information about mechanical performance of their tanks. Shermans in Africa had their tracks last for about 600 miles, after which the rubber would begin to peel. Some units had to drive around on peeling tracks since replacements were hard to come by. Of course, driving around in heat and over rocky terrain is probably the worst way to treat rubber, so it's not a fair comparison.

There were also other periodicals regarding vehicle performance, such as Vehicle Review. Edition #15 for April 1945 notes poor performance of T74 tracks at high speeds: "One set of T74 track has been judged unserviceable after 261 miles. Following a high speed road run for bogie tire temperatures, the tracks became overheated to such an extend that most of the rubber pads blew." Enforcement of low speed limits when outside of combat was important in order to extend the lifespan of rubberized track links. I have never encountered any documents setting such a speed limit for CDP tracks.

A clear advantage of CDP tracks was that they were lighter. A full set of T54E1 tracks weighed 3520 kg, a full set of CDP tracks weighed 2086 kg.

Field modification to Canadian Dry Pin tracks used on Sextons to extend their longevity.

In combat, heavy idler wear (chiefly on Sextons) was reported. This was because mud would pack on the inside of the track and stress the idler bearing. The metal track links were also tougher on the rubber rim of road wheels and idlers (doubly so for synthetic rubber) than rubber coated tracks. To combat this, three Ram tanks with CDP tracks were tested. One had a sprocket with longer teeth to prevent track jump, which allowed the track to be loosened. The other two tanks were not modified. When run through heavy mud, the control tanks broke after 14 and 18 miles respectively, while the improved tank drove for 150 miles of heavy mud ("soft mud, high sand and gravel content, hard bottom") without trouble. The sag did not seem to impact performance at all. Crews felt that their tanks could go where Shermans with steel chevron tracks could not. However, the Shermans had one advantage: they could fit Extended End Connectors, which improved their flotation characteristics, while Sextons that used CDP tracks could not.

By May 4th, 1945, it seemed that these tracks were still considered acceptable in heavy mud. Instructions for field conversion were distributed to the troops "for use when terrain causes a necessity".

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