The actuator type on a motor-driven dosing pump determines what the pump does in the absence of a drive signal or in the event of a power failure. This is not a minor technical detail. In chemical dosing applications, the consequence of a pump running when it should not — or stopping when it should not — can range from chemical overdose and product contamination to inadequate treatment and downstream system failure. Understanding the difference between spring return and non-spring return actuators, and choosing the correct type for the application, is a fundamental part of dosing system design.
How a Motor-Driven Dosing Pump Actuator Works
In a motor-driven diaphragm or piston dosing pump, the actuator converts the rotational motion of the motor into the reciprocating motion that drives the diaphragm or piston back and forth to produce the pumping action. The stroke rate — how many times per minute the diaphragm reciprocates — is controlled by the motor speed or by a control signal. The stroke length — how far the diaphragm travels on each stroke — determines the volume displaced per stroke.
The actuator type determines what happens to the diaphragm or piston position when the motor is de-energised or the control signal is removed.
Spring Return Actuators
A spring return actuator incorporates a mechanical spring that acts against the motor drive. When the motor is energised, it works against the spring to drive the diaphragm through its stroke. When the motor is de-energised — whether due to a power failure, a control signal loss, or a deliberate shutdown — the spring returns the diaphragm to a defined rest position.
The critical characteristic of a spring return actuator is its fail-safe behaviour: the pump returns to a known, predictable state on loss of power or signal. In most configurations, this rest position closes the pump — the diaphragm returns to the position that minimises or stops flow. This is fail-closed behaviour, and it is appropriate for applications where dosing must stop in the event of a power or signal failure to prevent overdosing, contamination, or safety incidents.
Examples include biocide dosing in food processing water, acid or caustic dosing in pH control loops, and chemical injection in applications where an uncontrolled dose carries a safety or quality risk. If the control system fails, a spring return pump defaults to not dosing — which may be suboptimal for treatment but prevents a worse outcome.
The trade-off with spring return actuators is that the spring force adds to the mechanical load on the motor during the drive stroke, which increases power consumption and can affect stroke linearity at the extremes of the stroke range.
Non-Spring Return Actuators
A non-spring return actuator has no restoring spring. The motor drives the diaphragm in both directions — the forward stroke and the return stroke are both motor-driven. When the motor is de-energised, the diaphragm stays wherever it is. There is no defined rest position and no inherent fail-safe behaviour.
The advantage of non-spring return actuators is mechanical efficiency. Without a spring to work against, the motor drives only the load imposed by the process and the diaphragm itself. This allows for more linear stroke behaviour across the full stroke range and lower power consumption at equivalent duty.
Non-spring return actuators are appropriate for applications where the pump position at rest is not safety-critical — where stopping in a mid-stroke position causes no harm, and where the control system is reliable enough that power or signal loss is not a significant failure scenario to design against. They are also appropriate in applications where the pump is integrated into a system with its own shutdown and safety logic that does not depend on the pump actuator for fail-safe behaviour.
Choosing Between the Two
The starting point for actuator selection is a failure mode analysis: what happens to the process if the dosing pump stops unexpectedly, and is that consequence acceptable? If stopping the pump in an undefined state could cause a safety incident, a quality failure, or a significant process upset, spring return fail-closed behaviour is indicated. If stopping in an undefined state is acceptable because the consequences are manageable and the control system provides adequate protection, non-spring return is viable and offers mechanical efficiency advantages.
A secondary consideration is control signal type. Spring return actuators are well suited to on/off control — the spring provides the return, and the motor provides the forward drive in response to a signal. Non-spring return actuators are more naturally suited to continuous modulating control, where the motor drives both directions in proportion to a control signal such as 4-20mA or PWM.
The Injecta range of motor-driven dosing pumps covers both actuator types across diaphragm, piston, and digital motor-driven configurations, allowing the correct fail-safe behaviour to be matched to the application. To discuss actuator selection for your dosing application, contact us at info@autoflotechnology.com.