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How Thermal Actuators Work

Converting Energy to Motion

Wax based thermal actuators operate on the principles of thermal expansion. As wax is heated and changes from a solid to liquid its volume increases approximately 15 percent. In turn, we are able to convert heat energy into a linear mechanical motion. In use for over half a century, thermal actuators have proven to be a durable and reliable source of converting heat energy into mechanical energy while doing so in a compact package.

how thermal actuators work

Keystone thermal actuators are precisely calibrated to produce the exact specifications required from the customer.

Operating Temperatures

Operating temperatures can be specified by the customer from 60 to 250 degrees Fahrenheit.

Thermal Recipes

Thermal Actuators are powered by a charge of parafin wax assisted by other elements to improve heat transfer.  These charges are custom designed to operate at the desired temperature.  Keystone works directly with our wax manufacturers to develop the properties needed for each application.


Once we have a charge that operates at the desired set point, we must calibrate the thermal actuator to ensure proper stem height.  This is done by controlling the temperature of the actuator and creating “dimples” in the sides or bottom of the cup. This dimpling effect raises the stem until it reaches the desired stem height.


One of the design considerations is hysteresis.  Depending on whether the actuator is heating to a liquid or cooling to a solid the stem height will be different at the same temp.  Basically there is 5 degrees hysteresis in the products.


  • Compact
  • Durable
  • Reliable
  • Not affected by vibration
  • Works with liquid or gas
  • Does not require electricity
  • Customized for your product
  • Dedicated Keystone Engineer
  • Reproducible, re-order anytime
Thermal Actuator
Control Valve for a Thermal Actuator
Spring Return Assembly
Finished Product cross section

Please give us a call for more information!

All our products are built to order.  Call one of our engineers to get started.