The Internal Structure Types of Thermal Actuators & Wax Motors

Posted on December 13, 2021

Based on different requirements for stroke, performance, and external dimensions, the internal structures of wax motors and thermal actuators can be divided into three types: flat-diaphragm type, squeeze-push type, and plunger type.

Exploded view for Thermal Actuator

Flat-diaphragm type

It is the most common type of wax thermostatic element. Since the deformability of the diaphragm is constant, the flat-diaphragm type thermal actuator is only suitable for applications that don't require long strokes. The output stroke of the flat-diaphragm type thermal actuator is the shortest among the three, generally not longer than 4mm.

In the meanwhile, as the piston is on the top of the diaphragm, the longer guide for the piston is a must.

A Gif to demonstrate Diaphragm-type wax motor

Squeeze-push type

It comes to this type when there are additional requirements for long-term reliability, higher stroke, and smaller size.

In this type, we insert the piston in an "elastomer bag" surrounded by wax. As the wax expands, the "bag" squeezes and pushes the piston to produce displacement. And as there is the frictional force between the "bag" and the piston, the squeeze-push type thermal actuator requires a higher external reset force to have the piston return to its initial position.

The height of the squeeze-push type thermal actuator can be shorter than that of the flat-diaphragm type thermal actuator because the squeeze-push type thermal actuator doesn't require a long guide for the piston. The service life cycles of the squeeze-push type can reach 100,000 times and the output stroke can be higher because the deformability of the "elastomer bag" is much better than the diaphragm.

iSwell only manufactures squeeze-push type thermal actuators.

A Gif to demonstrate Capsule-type wax motor
Working principle of a thermal actuator

Plunger type

The plunger type wax motor can produce the highest stroke because it's not limited by a diaphragm or "elastomer bag". The piston is directly inside the vessel filled with wax. With the same output stroke, the size of the plunger type wax motor is the smallest among the three. But as there is no diaphragm or "bag" to keep the melted wax inside the vessel, this type of wax motor has much more critical requirements for sealing materials and processing accuracy.

The wax motor in iSwell P21 thermoactuators is a plunger type wax motor.

A Gif to demonstrate Plunger-type wax motor

Bottom Line

Here are our suggestions on how to select wax thermostatic elements from these three types.

• If the required stroke is higher than 4mm, plunger type or squeeze-push type is the better option.
• If the space is limited in the application, plunger type or squeeze-push type is the better option.
• If the long-term durability is the biggest concern, squeeze-push type is the best option.

Back to blogs Page ➞
You Might Also Like
Introduction to Thermal Actuators

Exploring the functions and benefits of thermal actuators and their role in the underfloor heating systems.

read article ➞
Introduction to Thermoactuators

More information on Thermoactuators, including their definition, how they function, and tips for selecting the appropriate one.

read article ➞
Introduction to Wax Motors

Explaining the basics of a wax motor and the capabilities of wax, and exploring the various applications where wax motors are utilized.

read article ➞