Land mines have been a part of warfare since the 13th Century and were ever-present on the far-reaching battlefields of World War 2 (1939-1945). The threat it posed to tanks and infantry alike was such that many measures were taken to overcome these unbiased buried explosives. One key development of the war period - credited to South African Major A.S.J. du Toit - was the "chain flail" system mated to the hull of an existing tank.
In the du Toit approach, a rotating drum was set some distance ahead of the tank by way of support arms and this drum was arranged with a series of heavy chains. The drum rotated under external power causing the chains to "flail". The resulting action was found to be enough to detonate most buried land mines and the Allies took this idea initially to British Matilda tanks in the North African Campaign of 1942 - producing the name of "Matilda Scorpion" for these vehicles. They first saw service in the designated role during the Second Battle of El Alamein (October 1942). In time, the flail system was applied to British Valentine and American M3 Grant tanks resulting in the "Valentine Scorpion" and "Grant Scorpion". In these applications, the turret (or main gun armament) of each respective tank was deleted and an external power supply required to drive the flail drum.
However, as sound as the concept was, further work was required to streamline the mechanism for it was found through practical use that the flailing chains could become entangled with one another and reduce the effectiveness of the rolling unit as a whole. The system was also limited when used on uneven terrain as the fixed-length chains could only reach so far, leaving pockets of terrain untouched with the possibility of live mines waiting underneath.
British engineers took the concept and refined it into the "Crab". This design used forty-three or so individual strands of chain, again fitted to a rotating drum, and seated ahead of the advancing vehicle by way of support arms. Drive power was now taken from the tank's engine itself so no external power supply was needed. Other qualities were eventually built into the Crab's design including a contour-following capability for a more even flailing spread of the chains (particularly over uneven terrain) and better blast protection for the host tank. A barbed wire cutting feature was also added in time.
The most popular Crab application arrived with the widespread use of the American M4 Sherman Medium Tank. The tank was produced in the tens of thousands during the war period and, as such, was available in considerable numbers despite its limited battlefield effectiveness when compared to some of its direct contemporaries. Regardless, it was a serviceable solution and logistically-friendly by sheer numbers alone. The Crab system was applied to the Sherman series to produce the "Sherman Crab" mine flail tank.
The Sherman Crab saw first-service with British forces (79th Armored Division) before the U.S. Army received its initial examples. The Sherman-based Crab became the most widely-used mine flail tank of the entire war. The tank's original drivetrain was proved capable for the Crab/Sherman mating and the tank was further allowed to retain its useful turret - thus its offensive capabilities were left largely untouched. The flail unit did extend the overall length of the vehicle considerably which limited its turning radius as well as ditch-crossing ventures. Regardless, this sacrifice was worth the return-of-investment as mine fields could be dealt with ahead of the main supporting force through this cost-effective measure. Two Sherman Crab marks were eventually produced as the "Sherman Crab Mk I" and the "Sherman Crab Mk II" - these built atop different Sherman production models.
Sherman Crabs were deployed during the Allied advances of 1944-1945 in Western Europe. In U.S. Army service, the Sherman Crab Mk I received the designation of "Mine Exploder T2 Flail" and the Sherman Crab Mk II became the "Mine Exploder T4".