Automation & Motion Control

Optimization of Total Operating Costs Using Electric Linear Drives

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LinMot USA
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LinMot USA   5/8/2014 4:27:32 PM
Figure 4 on the second page of the article shows the sizing diagram for ISO standard cylinders from the market leader in pneumatics.  As this figure shows, the comment of "maxthrottle" is correct for static or very slow movements.  For more dynamic movements with higher piston speeds (our example requires 1m/s), bigger bore diameters are needed.  In our example with 15kg payload and 1m/s the sizing diagram (Figure 4 on page 2) for a standard ISO cylinder indicates a 50mm bore diameter cylinder.

According to the calculation and the measurements on a real machine, the pneumatic cylinder in this example has 30 times higher energy requirement compared to the electric solution with an Industrial Linear Motor.

Now let's assume that the same application could have been done with a smaller 25m bore diameter cylinder (let's just assume, although figure 4 shows that it would not be possible at the required speed).  With only halve the bore diameter, the air and therefore the energy consumption of the pneumatic cylinder could theoretically be reduced to 1/4 of the calculated value with the correct sized 50mm bore diameter cylinder.  This means that the smaller 25mm pneumatic cylinder would still have a 7.5 times higher energy consumption compared to the linear motor solution.

Even with a smaller bore diameter pneumatic cylinder, the energy consumption is well above the electric linear motor solution.  This is the point of the article.  If you desire pneumatics for medium to high performance actuator applications, be prepared to pay a lot more money to operate your machinery.  In addition, it helps to know that an electric linear motion solution requires minimal to no maintenance and increases plant up time considerably.

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This math is way off !
PhilONeill   5/8/2014 2:15:46 PM
First, I'll admit to working for a pneumatics company so I have a bias :-)

This didn't pass the sniff test so I looked a little deeper.  This article has a lot of givens that are suspect.  Using pneumatic simulation tools, I find that the selection of a 50mm cylinder is too large, a 40mm is adequate, either piston rod type or rodless, and at 5 bar, not 6, it is still possible to reach the 400mm in 500ms target.

Secondly, the math is way off.  Again, using tools from our website, I estimate the air consumoption for a rodless cylinder (which has higher consumption than a piston rod type) 30 cycles per minute, 8000 hours per year to be just over 100,000 LITERS per year, not 150,000 CUBIC METERS per year.  Since there are 1,000 liters in a cubic meter, that would be 100 cubic meters per year, not 150,000.

And it's not just the air calculations.  The figure of 0.13 kWh per cubic meter to compress to 6 bar seems too good to be true, making the cost per liter $0.00013.  That would mean the 100 cubic meters of air needed would cost $13 (~10 Euro)per year, and I'm sure that's not right either.


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Savings of Electric Actuator vs. Pneumatic are Overstated
maxthrottle   4/16/2014 3:58:06 PM
Although electric actuators can be more efficient than pneumatic actuators, This author has overstated the savings of using an electric actuator vs a pneumatic actuator.

The autor suggests it is necessary to use a 50mm bore air cylinder at 6 bar to move a 15Kg load.  An actuator of this size will move 128 Kg, 8.5 times more than the load!  If the electric actuator was sized with a 850% safety factor I would imagine the pneumatic actuator would have the advantage over the electric actuator.  Pneumatics as a automation solution is falling out of favor due to many economic reasons but it still places a huge amount of power in a small package without the worry of electrical concerns.   With fast cycle rates and light payloads, and over the long term, there is often a good argument and economic advantage for using electric actuators vs. pneumatics.  Why do so many electric actuator manufacturers find it neccessary to distort the economic advantages of electric actuators? 

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