What are pneumatic cylinders with rod lock?
Pneumatic locking cylinders – also referred to as cylinders with rod lock - are pneumatic actuators equipped with a mechanical or pneumatic locking unit designed to hold the piston rod in a fixed position during pressure loss or intentional stops. These devices are essential in applications where position retention, operator safety, or load security is required.
SMC’s locking cylinders incorporate spring-actuated locking mechanisms or pneumatically released locks, ensuring that the actuator maintains its position when needed — in the extended, retracted, or both directions depending on the series and configuration selected.
Locking direction options
Depending on the series and bore size, SMC locking cylinders are available in extension locking, retraction locking, or — in some configurations — both directions. The locking direction must be specified at the time of ordering and cannot be changed in the field.
Extension locking holds the piston rod against extension loads. This is the correct choice when the load tends to push the rod outward under gravity or spring force — for example, a raised platform or clamped workpiece that would otherwise move further outward when pressure is lost.
Retraction locking holds the piston rod against retraction loads. This is used where the load acts to pull the rod inward — for example, a vertically mounted actuator supporting a suspended load that would otherwise retract when pressure is lost.
It is important to note that unless a bidirectional configuration is selected, a locking cylinder generates holding force in one direction only. The extension lock provides no resistance against retraction loads, and retraction locking provides no resistance against extension loads. Selecting the correct locking direction for the application is therefore critical.
SMC Locking Cylinder range
SMC offers one of the broadest ranges of pneumatic cylinders with rod lock on the market, covering bore sizes from Ø20 mm to Ø100 mm across multiple series. The following are just a few examples from the full product range:
A double-acting cylinder with integrated lock unit, available in bore sizes Ø32 to Ø100 mm. The lock unit is separable from the cylinder body for easier maintenance and replacement. Locking is bidirectional, with holding forces from 630 N (Ø32) to 6,080 N (Ø100). A built-in manual lock release allows the locked state to be held without pressurising the unlock port, simplifying installation and adjustment.
A compact, guided actuator combining the precision of SMC's MGP guide cylinder with an integrated lock unit. Available in bore sizes Ø20 to Ø100 mm with slide bearing or ball bushing guide rod options, the MLGP is the ideal choice for applications involving lateral loads.
A unique combination of integrated air cushion and locking unit in a compact body, available in bore sizes Ø32 to Ø63 mm. The air cushion absorbs impact at stroke ends and reduces impulsive noise, making the RLQ particularly well suited to higher-speed applications where both cushioning and drop prevention are required. By-pass piping for drop-prevention circuits is available factory-fitted.
Across all series, piston speeds up to 500 mm/s and operating pressures up to 1.0 MPa are supported, with holding forces ranging from 157 N (Ø20) to 3,927 N (Ø100).
Typical applications of pneumatic locking cylinders
Pneumatic cylinders with locking unit are used in a wide range of vertical applications for drop prevention, as well as in applications in which intermediate stops or emergency stops are required, such as:
- Workpiece drop prevention – the lock engages automatically when air pressure is lost, preventing tools or fixtures from descending under their own weight
- Workpiece clamping – retraction locking maintains clamp force on a workpiece even if system pressure is lost
- Main shaft balancer of processing machine – prevents drift on balanced axes when power or pressure is cut
- Lifter drop prevention — holds a lifted load securely in position during an emergency stop or pressure loss
- Safe brake control: the lock or brake unit is used for intermediate stop, emergency stop and drop prevention.
Key considerations when selecting the right pneumatic cylinder with rod lock
When specifying an SMC locking cylinder, engineers should evaluate the following parameters:
- Bore size and required holding force — Holding force varies by bore size and series. Note that the quoted holding force always represents the maximum static capability; apply an adequate safety factor for your application load.
- Locking direction — Extension, retraction, or bidirectional locking must be selected at order; it cannot be changed in the field.
- Mounting orientation and guide requirements — Consider whether your application involves lateral loads or moment loads, in which case a guided locking cylinder variant will be more appropriate than a standard rod-only configuration.
- Stroke length — Standard strokes are available across the range; intermediate strokes can be configured via spacer installation on many series. Refer to individual series documentation for available stroke increments.
- Cushioning requirements — If impact absorption at the stroke end is required alongside position locking, consider a series that combines an integrated air cushion with the locking unit.
- Auto switch compatibility — Many series are available with built-in magnets for position sensing, compatible with SMC's full range of reed and solid-state auto switches. Refer to individual series documentation for compatibility details.
Contact SMC’s expert team to discuss your pneumatic locking cylinder requirements and receive tailored support for your pneumatic system design.
Frequently asked questions about pneumatic cylinders with rod lock
What is a pneumatic cylinder with rod lock used for?
A pneumatic locking cylinder is used wherever a pneumatic actuator must hold its piston rod position when air pressure is reduced, cut off, or intentionally exhausted. Typical uses include drop prevention on vertical axes, workpiece clamping retention, emergency stop position holding, and mid-stroke locking for safety.
Can an SMC locking cylinder be locked at any position within the stroke, or only at the end positions?
SMC locking cylinders can be locked at any position within the full stroke — this is one of their key advantages over simple end-position locking devices. This makes them suitable for mid-stroke emergency stops and applications where the locking position varies depending on workpiece dimensions.
Do SMC locking cylinders have a manual unlock function?
Yes, all SMC locking cylinder series include a manual unlocking provision for maintenance and emergency situations.
