Ni-Ti shape memory actuators respond to temperature changes with a shape change. Therefore, they are sensors and actuators. They combine large motion, rather high forces and small size, thus providing high work output. They usually consist of only a single piece of metal, e.g., a helical spring, and do not require sophisticated mechanical systems. They often fit into tight spaces in given designs, where other thermal actuators, like thermostatic bimetals or wax actuators, would require a major redesign of the product. In flow-control or oil pressure control valves, for example, helical springs can be placed in the fluid path, without restricting the flow. Thus, they provide fast response to changes in temperature. Shape memory actuators have been used successfully in the areas of thermal compensation, thermal actuation, and thermal protection.