Thermal Actuators

Caltherm's actuators are designed and constructed to perform perfectly per the customer's requirements and are used in many applications across multiple industries. Thermal actuators convert temperature change into a mechanical force to push/pull, open/close or move a load. Thermal actuators are the "engines" of thermostats, oil valves and temperature relief valves.

Operating Principles

Expansion material is carefully formulated to meet the required characteristics of each application in order to convert changes in temperature into mechanical motion. Caltherm designs its thermal actuators using one of two main approaches – flat diaphragm or squeeze-push – to generate piston movement when exposed to heat.

Flat Diaphragm Element

The precise movement obtained by the flat diaphragm technology makes it the ideal choice for many applications. The approach results in longer life and less stoke loss over numerous cycles. The diagram below illustrates how the flat diaphragm elements functions.

When exposed to heat, the expansion material, enclosed in the cup , expands and pushes against the diaphragm, the movement is transmitted via the plug to the piston. The guide secures the diaphragm sealing the wax inside the cup and allows the plug and piston to slide freely. An external spring (not pictured) ensures the return of the piston when cooling down.

Diaphragm Actuator Design

Squeeze-push element

Squeeze-push technology is used when additional stroke is required and accuracy can be sacrificed. This technology is also utilized when available space is a concern.

In this design, the piston is surrounded by an "elastomer bag." When heated, the wax , enclosed in the cup, expands and applies pressure, via the bag on the piston with both a radial force ("squeeze") and an axial force ("push"). An external spring ensures the return of the piston when the temperature decreases.

Squeeze-Push Design

Performance Curve

The illustration below shows the precise piston position over the operating temperature range of a thermal actuator. The rightmost curve shows the power stroke, during which the wax expands as the temperature increases. The leftmost curse shows the return stroke, during which as the temperature decreases the wax contracts. The difference between the two curves is called hysteresis and is caused by the compression and friction of the internal components and thermal inertia. The shape of the curve and its gradient, α (mm/˚C or inch/˚F), depend on the proprietary formula of the wax blend.

Typical Performance Curve

Caltherm Solutions

Caltherm offers several different sizes of thermal actuators in both flat diaphragm and squeeze push designs. Each design is available to meet the specific customer application requirements. Please see Table A to view of summary of sizes for each technology.


Style A B C D E F
Flat Diaphragm/Squeeze Push FD FD & SP FD FD & SP FD FD
Max OD 10mm 13mm 15mm 18MM 22MM 33mm
STM position (Typical) 0.590 0.295 0.597 0.769 1.792 2.517
Min STM Temp 64 - 185 100-194 60 - 208 60 - 205 85 - 210 110 - 160
Max Full Open Temp 117 - 220 120-212 80 - 226 125 - 220 110 - 230 130 - 180
Net Stroke (min. -max.) .085 - .120 .287 - .410 .097 - .145 .315 - .385 .190 - .315 0.410
Spring Load (Typical) 5.5 lbs 6.0 lbs 10 lbs. 15 lbs. 23 lbs. 78 lbs.
Spring Rate (Typical) 14 lbs 20 lbs 63 lbs. 30 lbs. 45 lbs. 70 lbs.