Fall Protection Design of Air Cylinders｜Practical Guide

Here, it is inexpensive and easy to incorporate into an automatic machine."Techniques for preventing air cylinders from falling."　notes.

When designing automatic machines that lift and lower vertical shafts, many designers are concerned about measures to prevent workpieces from falling when the air supply is interrupted.　　General explanatory websites often only introduce closed-center solenoid valves and cylinders with locks, and do not cover the detailed risks faced in practice, such as minute drops due to air leaks and violent shocks when restarting.

This article systematically explains the physical fall hazard in a vertical axis, the unique characteristics of pneumatics, and the circuit configuration and adjustment methods to prevent ejection during restart and ensure safe operation.

We hope that this article will help you make design decisions based on the evidence of why the procedure is necessary.

Air Cylinder Fall Protection and Basic Theory

Gravitational potential energy and fall hazard

When dealing with vertical axes in machine design, the first thing to consider is the magnitude of the potential energy possessed by the workpiece.　　A driven object in a high position stores energy, represented by the equation E = mgh, which turns into a destructive force the moment the power source is lost.

If an air source cutoff occurs, this energy is immediately converted to kinetic energy and begins free-fall.　　This is because the fall velocity v increases in proportion to the square root of the fall distance h,Even if the stroke is short, the impact force at the time of collision cannot be ignored.　Understanding these physical characteristics is the foundation for the designer's initial risk assessment.

Air compressibility and creep phenomena

One of the factors that make air cylinders difficult to control is the compressibility of air and the creep phenomenon.　　Unlike hydraulic pressure, air easily changes volume in response to external forces, so even if air is trapped inside the cylinderThe workpiece bounces and sinks like a "spring."

In particular, it is impossible to completely eliminate minute air leaks from packings and valves.　　If the holding pressure is lost over time, the piston will not be able to resist gravity and will continue to slowly descend by a few microns.To prevent this, not just pressure containment, but in conjunction with a physical retention mechanism is a realistic option.

JIS B 8370 (General rules for pneumatic systems)

An official standard, JIS B 8370 (General Rules for Pneumatic Systems), provides guidance for pneumatic design in Japan.　　This standard identifies the major sources of danger associated with pneumatic systems and provides detailed principles for avoiding them.

Specifically, the system must maintain a state of non-hazard to personnel in the event of loss of power source or pressure drop.　　For example, they stress the importance of designs that retain load even when energy is shut off and mechanisms that prevent unexpected restarts.　　Compliance with these requirements is,Minimum rules for fulfilling your responsibilities as a designer.It can be said to be.

Occupational Health and Safety Regulations, Article 101 and Article 526

As legally binding standards, it is essential to understand Articles 101 and 526 of the Occupational Safety and Health Regulations.　　Article 101 of the regulations requires that rotating shafts and moving parts that may pose a hazard to workers be covered or enclosed.Vertical shaft lifting areas are also included in this provision and are subject to physical protection measures.

Article 526, on the other hand, provides for the provision of safe lifting equipment when working in areas where the height exceeds 1.5 meters.This concept also applies to safety during maintenance. For equipment that may fall unexpectedly, the risk of workers entering under moving parts must be eliminated to the greatest extent possible.　　Designing for compliance with regulations provides a bulwark to protect a company's legal liability in the event of an accident.

Reference source: JIS B 8370:2013
Reference source: Health and Safety Information Center (https://www.jaish.gr.jp/anzen/hor/hombun/hor1-2/hor1-2-1-2h9-0.htm）

Advantages of Locking Cylinders and Preventing Air Cylinders from Falling

Fail-safe and Intrinsically Safe Design

The core of the safety design is,Fail-safe and Intrinsically Safe Design Concepts　The idea is to integrate the　　Intrinsically safe design refers to a design that eliminates the source of the hazard itself, while fail-safe refers to a mechanism that always operates on the safe side in the event of a failure.

In preventing air cylinders from falling, locking cylinders are the embodiment of this fail-safe device.　　By selecting a model that "locks" when air is shut off, any system malfunction is directly linked to safe retention.　　Thus,A thought process that ties the loss of energy to a safe state of "stillness"　creates a highly reliable design.

Operating principle of the spring lock system

The spring-locking actuation principle used by most lock cylinders provides excellent physical certainty.　　This mechanism has a strong spring inside the cylinder, and the rod is strongly gripped by this spring force during normal operation.　　In other words, the basic specification is that the brake is applied when nothing is being done.

To operate, air is supplied to the release port to push the spring back, thereby unrestraining the rod.　　If the piping breaks and air is lost, the spring force is immediately restored to hold the rod in place,Workpieces can be reliably prevented from falling even in the event of a power failure.　　This physical holding force, which does not depend on the compressibility of air, is the main reason why it is possible to hold a position for a long time.

Piston rod rotation torque prohibition during lock operation

The absolute prohibition that must be observed when using lock cylinders is the prohibition of piston rod rotation torque during lock operation.　　The holding mechanism is secured by crimping the rod surface with brake shoes, etc. If a rotational force is applied to the rod in this state, the internal precision holding parts will twist and break.

Rotational torque not only causes a significant reduction in holding force, but also scratches the rod surface and shortens the life of the packing.　　Therefore, if there is a possibility that the load may include a rotational component, the cylinder must be designed so that only an axial load is transmitted to the cylinder by an external non-turn guide.　　The secret to long-term operation is to treat the cylinder as a mere power source and to avoid placing an unreasonable load on the holding mechanism.

Combined use of external stopper and fall prevention pin

Rather than relying solely on the cylinder's built-in lock, a combination of an external stopper and anti-drop pin is also worth considering as an additional safety measure.　　This is because double and triple protection is required, especially in vertical axes that handle large masses and in high-risk areas where workers can get under the machine during maintenance.

There is no more reliable measure than a physical stopper to limit the stopping position and a mechanism to insert a safety pin into it.　　Recently, the standard approach is to use an air chuck or air cylinder to automatically insert and remove pins, and to assemble an interlock that monitors their position with a proximity sensor.Keep in mind that if one function fails, another function will complement it in a redundant design.

Residual pressure venting and safety procedures during maintenance

A potential design blind spot is the construction of procedures to vent residual pressure and ensure safety during maintenance.　　It is not uncommon for a locking cylinder to leave high pressure air trapped in the circuit.If the piping is disconnected without considering this residual pressure, a pinching accident may occur due to unexpected cylinder movement.

To prevent this, it is important to install a "manual residual pressure exhaust valve" as standard equipment at the design stage.　　Ensure that the air source is shut off before work, and that the pressure gauge confirms that the residual pressure has been completely released before work begins.　　It is the kindness required of designers to provide a means of releasing residual pressure as hardware, rather than relying on personal attention for safety.

Reference source: SMC Corporation (https://www.smcworld.com/upfiles/_manual/j/CEx-OMG0103.pdf）

Reference source: CKD Corporation (https://www.ckd.co.jp/kiki/jp/file/6309）

Vertical axis Drop prevention Circuit design prevents air cylinder from falling

Pressure holding by pilot check valve

It is often used to prevent relatively lightweight workpieces from falling or as an intermediate stop aid.Pilot check valve　This is pressure retention by　　This is a type of check valve that is mounted directly on the port of the cylinder to open the exhaust only while air pressure is being supplied.　　When the air source is shut off, the valve closes immediately, trapping air in the cylinder.

However, as mentioned above, air is not immune to microleakage,Not suitable for holding still for long periods of time.　　Even so, the ability to stop air in the immediate vicinity of the cylinder makes it an excellent choice as an initial measure to prevent sudden drops, such as when a tube breaks.　　Depending on the application, consider using a physical lock or a combination of locks.

Utilization of SSC valves (start-up speed control valves)

In the vertical axis, a phenomenon that is as alarming as falling is popping out during restart.　　The trump card to curb this is,SSC valve (start-up speed control valve)This is the utilization of the　　This valve has the ability to throttle the supply in meter-in control at startup when there is insufficient pressure in the cylinder, and automatically switch to normal meter-out control after sufficient pressure has been built up.

This mechanism allows the piston to start moving slowly even when starting up from a state of no residual pressure.　　The ability to maintain takt time during normal operation while preventing personal injury or product damage is a major advantage. We recommend that this system be incorporated as an essential component in the design of vertical shafts to ensure safety.

Optimization of meter-in meter-out control

To stabilize the operation of the vertical axis,Optimization of meter-in meter-out control　is extremely important.　　Generally, the speed of an air cylinder is adjusted by meter-out, but since the piston tends to run out of control due to gravitational acceleration in a vertical axis descent, a technique to increase "air rigidity" by also squeezing the supply side is also theoretically possible.

The adjustment procedure is to first adjust the meter-out side until a stable speed is obtained under no load, and then gradually close the meter-in side.　　If the needle is fixed at the point where the speed begins to drop slightly, both chambers of the cylinder are always filled with air, enabling stable lifting and lowering that is resistant to load fluctuations.　The key to smooth operation, however, is the rigidity provided by this "double-sided squeezing,In the first place, I personally do not recommend trying to control falling workpieces with compressible air.

Allowable kinetic energy and shock absorber selection

Allowable kinetic energy and shock absorber selection are the final bulwark when fall protection measures fail or when speeds exceed the control range.　　The cushioning capacity of the cylinder body is limited, and the energy E = (1/2)mv^2, proportional to the square of the mass m and the velocity v, must be calculated exactly.

If the calculated energy exceeds the cylinder's capacity, an external hydraulic shock absorber should be installed to absorb the shock.　　For vertical shafts, the ironclad rule of safe design is to estimate a higher than normal maximum speed, taking into account the acceleration due to the machine's own weight.　　Complete the structure so that the entire machine can safely absorb the shock and protect the equipment in the event of a collision.

Pressure balance circuit when unlocked

To maximize the potential of a cylinder with lock, it is essential to design a pressure balancing circuit for unlocking.This is because if the lock is released when the pressure is unbalanced between the upper and lower chambers of the cylinder, the piston will move abruptly toward the lower pressure side, causing a shock.

Therefore, a sequence may be constructed to supply equal pressure to both the upper and lower chambers in advance, just before sending the signal to release the lock.　　Once you confirm that the pressures are balanced, you can release the lock and the workpiece will not sink or bounce under its own weight and can quietly begin to operate.　　I have not done this technique, but try to incorporate it if necessary.

Summary (Preventing air cylinders from falling)

• Important points regarding fall prevention of air cylinders are summarized.
• The vertical axis always holds enormous positional energy due to gravity
• Due to the compressibility of air, mere closure is not enough to completely prevent micro-falls
• Design prerequisite for safety maintenance in case of abnormality in accordance with JIS B 8370
• Provide appropriate protective measures for moving parts in accordance with the provisions of the Occupational Safety and Health Regulations.
• Select a locking mechanism that activates in the event of air loss for fail-safe reasons
• Spring lock system can physically hold workpieces even when power is lost
• Internal damage occurs when rotating torque is applied to the piston rod during lock operation.
• For applications where there is a risk of rotation, install external guides to protect the cylinder body.
• Use external stoppers and safety pins in critical areas to create multiple layers
• Before maintenance, use a residual pressure exhaust valve to reliably relieve pressure in the circuit.
• Sudden startup events can cause catastrophic damage to equipment.
• Utilize SSC valve to limit supply flow only at startup to prevent pop-out
• Appropriate combination of meter-in and meter-out controls to increase rigidity
• Before unlocking, pressure is supplied to both cylinder chambers for balancing.
• Strictly calculate allowable kinetic energy and compensate for shortfalls with shock absorbers.
• Design to protect human life and equipment by taking thorough measures to prevent air cylinders from falling.

That's it.