The M5A1 Stuart light tank (Figure 1) uses the T19 rubber track. Since this type of track has not been manufactured recently, new old stock (NOS) is often used as a replacement. The problems with NOS track are usually attributed to aging of the rubber. Rubber is a natural polymer that is well suited to tank track, even by today's standards. However, over time, the long rubber molecular chains cross link with oxygen atoms resulting in stiffening of the structure and loss of compliance. As this aging occurs, new track blocks often exhibit polymer failure modes characteristic of hundreds of miles of use when only driven a few miles. Figure 2 is a view of this condition, characterized by gouge marks and holes in the track. Chunks of rubber are gouged out during steering of the vehicle on surfaces such as asphalt or concrete. If a vehicle makes a 90 degree turn, the track blocks at the front and back scuff at high speed resulting in such material failures. Track blocks near the pivot point (middle) usually fare better. Finally the track wears to the connecting pins as shown in Figure 3. Then the vehicle starts riding and wearing the pins and connectors, further accelerating track damage. Also the ability of the vehicle to leave an asphalt road undamaged is reduced since metal now rubs on the asphalt surface.
Over the years several ideas have arisen to rehabilitate tank track by either cleaning off the old rubber and recasting new track or making new track from scratch, a costly process.
An idea that may have merit is based on the premises that if one has a set of NOS track that has been moderately used, then perhaps periodic coating of the ground surface of the track block would reduce or eliminate wear.
Recently, some testing was performed using a polyurethane coating on 1944 vintage block track. Figure 4 is a view of a track block with the polyurethane coating applied. The coating has the viscosity of putty shortly after mixing, making it easy to apply and hold shape while curing. Figure 5 shows the putty being applied. The coating is available in a variety of colors but black is most appropriate since it matches the original rubber closely, after a slight amount of usage. Figure 6 shows the approximate coating pattern used on a test vehicle during 1994 and 1995. The original rubber track had about one half of its useful life remaining. The rubber track was prepared for coating by blasting with compressed air to remove dust and debris in cracks. Since the original rubber surfaces were already quite rough and since the effects of a cleaner/primer coating could not be assessed on old rubber, no additional surface preparation was used. Some surface preparations may further deteriorate the old rubber which tends to negate the effect of the polymer coating. The vehicle was kept in a shed while the upper tracks were coated. After about 48 hours of curing, the vehicle was moved forward to coat more track blocks that were not initially exposed. Figures 7 and 8 show views of the test vehicle tracks with approximately 1 mile of operation on clay soil and gravel surfaces. It should be noted that driving the vehicle on soil or gravel quickly removes the sheen, resulting in the more realistic surface roughness shown in Figures 7 and 8. After a few miles of operation, the polyurethane surfaces look nearly the same as the original rubber. Steering and track patterns on soil and grass appeared to be unaffected by the coating.
Wearing of the polymer coating depends on vehicular usage. After attending about 10 events a year and driving (with many 90 degree turns) from 30-40 miles per year over asphalt, concrete and gravel roads, the coating usually shears at the original rubber interface and requires replacement about once a year. If the vehicle is driven very little and in a straight line, then coating deterioration is significantly less.
There are several brands of polyurethane coatings on the market. The particular brand used should be in a putty form (when mixed) so that the shape of the coating can be controlled prior to setup without the use of a mold. Typical engineering properties characteristic of an acceptable polyurethane coating material are hardness (Shore A) 80-90, tensile strength 3500-4500 psi, resilience 40%, and abrasion resistance (Taper index) 90-120 mg loss. The particular polyurethane system used in this test was URS 5685 manufactured by Forsch Polymer Corp, Denver, Colorado. It takes approximately 2 to 2.5 gallons of putty to coat a set of Stuart track. Cost is about $100 per gallon.
The application of the polymer coating has stopped rubber track wear such that the purchase of a new set of track is not necessary, provided one maintains the coating. Although the cost per gallon of polymer appears high, it is much more cost effective than purchasing a new set of track (about $6000) if you can find it. It should be noted that the polymer putty can also be used to fill holes in road wheels as well as on half-track track. Although not a panacea, polyurethane coatings are a reasonable way to preserve old rubber track and put off the purchase of a replacement.