January 2011 Archives

Selection key points on air cylinders for LCA designs are organized and shown below.

(1) Checkpoints on Air Cylinder Selections

Cylinder force is determined by piston diameter and air pressure used. (See [Fig.1])

(2) Air Cylinder Force Calculation

(a) Checkpoints on Air Cylinder Selection - Dual Action Cylinders

Cylinder force is determined by piston diameter and air pressure used. (See [Fig.1])

(b)Single Action Cylinders

The value is a sum of dual action cylinder force and the internal spring force (for positive or negative pressure).

(3) Efficiency of Cylinder

The force generated with compressed air would be less than the theoretical force due to internal friction force, etc. Select cylinders based on cylinder force efficiency μ=approx. 50% for applied air pressure of 0.3MPa or more.

(4) Allowable Lateral Load

If lateral loads are applied on the piston rod, gouging can occur due to increased contact pressure on cylinder head bushing and cylinder tube inner wall. For max. lateral load limit, use 1/20 of cylinder max. force (μ=100%) for calculations.

In this discussion, pneumatic system component selection troubles and countermeasures are explained based on an example of "Unstable Actuation Time".

Following causes are conceivable for unstable air cylinder actuation time.

	1.	Supplied air pressure and volume
	2.	Increase of cylinder internal resistances
	3.	Load rate on air cylinders
	4.	Cushion function of air cylinders
	5.	External resistances and structural influences

The item 3. and 4. are explained here

(3) Problems caused by air cylinder load rates and countermeasures

The Load Rate is a rate of cylinder force capacity vs. applied load (W), and represented by the following formula.

When assimilating the instability of air cylinder actuation time with performance instability of automobiles, the cylinder force would be analogous to engine horsepower, and applied cylinder load to automobile weight. It can be understood that an automobile (air cylinder) must have appropriate power to weight ratio. The key point on LCA designing is to reduce (enlighten) applied loads.

Countermeasures for Load Ratio Related Problems

	・	Use air cylinders at as high a pressure as possible
	・	Select the largest cylinder bore diameter

(4) Countermeasures for Cushion Function Related Problems

There are air cylinders with and without cushions. In cases when a) applied loads are high, and/or b) operation speeds are high, the cushion function can degrade due to repeated usage and cylinder operation can become unstable.

(a) When the applied load is high

Kinetic energy can be obtained by the following formula. When E exceeds the air cushion capacity (shock absorber's max. energy absorbing capacity), a cylinder with one rank higher bore diameter should be selected.

(b) When the velocity is high

Same principle as the case of high loads. Select a cylinder with one rank higher bore diameter.

Countermeasure for Cushion Related Problems

・

Select a cylinder with one rank higher bore diameter for both High Load and High Speed cases.