Strap wrenches, commonly called chain wrenches, are used to hold or turn smooth circular objects, such as knobs, pipes and tubing. Strap wrenches work via a strap or chain that is pulled in tension around an object until it grips it firmly.
The strap or chain involved can take various forms. Some straps are made of nylon and are smooth and flexible, while others are steel, which are firmer but still with some flexibility. The chains tend to be of the roller chain type (like a bicycle chain).
Also available is oil filter wrenches designed for removing spin-on type oil filters. These filters are smooth, cylindrical canisters that are otherwise difficult to grip.
To use a strap wrench, place the end of the wrench with the teeth on it against the object you want to tighten, loosen or hold. Then wrap the chain around the object. Once the chain is attached, ratchet the wrench handle to move the object in whichever direction you need.
Strap wrenches are hand tools designed to use with difficult to grip or awkwardly shaped items such as pipe works, oil filters, pool filters, water filters, plumbing fittings, flywheels etc. Straps wrenches are mostly used in automotive and plumbing industries.