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Written by Techbricks.nl   
Tuesday, 29 April 2008 22:00

Pneumatic LEGO & laws of physics II
 

I have made measurements (pressure and force) on LEGO pneumatic cylinders. At this page I will explain the calculation of force caused by pressure and compare them with the measurements. Isn't it great to know what force a pneumatic LEGO cylinder can perform?

I have made an instrument to test the force that a LEGO pneumatic cylinder can perform as a result of air pressure.













I used a digital weighing device to measure to force. It can measure weight up to 25 KG (66.98 Pound) and it accurate up to 10 gram (0.32 ounce). May be it is a good idea for a new NXT sensor? A NXT sensor that can measure weight or force!

note: Most countries in Europe work with KG and gram in stead of Pound and ounce. Check http://www.convertworld.com/en/weight to convert the values to your most favorite system.
1 KG =
2.679 Pound 
1 Pound = 0.373 KG 

To measure the force that a pneumatic LEGO cylinder can pull or push, it is attached to one end of a lever. The lever is made of 8 technic LEGO liftarms 1 x 15. There are 4 on each site of the skeleton that is build around the weighing device. On the other end of the lever the weighing device is attached. At 6 holes from the right and 10 holes from the left of the lever there is a centre of gyration. Because it isn't in the centre, but on 6/10, the measured force is 10/6 higher then the force performed by the cylinder.
The hole framework around the weighing device and the lever itself must be very strong. The force performed by the pneumatic cylinder can reach several Kilogram's and otherwise the LEGO parts would bend or even break.

The force performed by the pneumatic cylinder can be calculate with the following formula:

Force  = Pressure x Area

Area
The area is the area of the cylinder piston. The bigger the area the bigger the force will be. LEGO pneumatic cylinders do have 2 chambers. The aera of the piston is bove chambers is different: When the piston is pushing the area is bigger then when it is pulling.  In Pneumatic LEGO and the laws physics I you can find calculations on several LEGO pneumatic cylinders.

note: Most countries in Europe work with metres in stead of inches. Check http://www.convertworld.com/en/length to convert the values to your most favourite system.
1 m (meter) = 100 cm (centimeter) = 1000 mm (millimeter)
1 m² = 10000 cm² = 1.000.000 mm²
1 inch =  2.54 cm
1 cm = 0.3937 inch
1 square inch = 6.4516 cm²
1 cm² = 0.1549 square inch

Pressure
The pressure is the air pressure within the cylinder. The bigger the pressure the bigger the force will be. Depending on the parts LEGO pneumatics can handle up to 28 - 35 PSI or 1900 -2500 mBar. I used a NXT and the Mindsensors Pressure sensor in combination with a NXT-G block that calculate the pressure in mBar.

note: Most countries in Europe work with Bar and mBar in stead of PSI. Check 
http://www.convertworld.com/en/pressure to convert the values to your favourite system.
PSI means pounds per square inch.
1 Bar (1000 mBar) = 14.5038 PSI = 100,000 Pascal
1 PSI = 0.068947mBar =
6894.75 Pascal

Force
A force (in general) can cause an object with mass to accelerate. It may be experienced as a lift, a push, or a pull. The acceleration of a body is proportional to the vector sum of all forces acting on it (known as the net force or resultant force). In an extended body, force may also cause rotation, deformation, or an increase in pressure for the body. Force can be calculated in Newton.

F  = m (mass) x g (gravity)

Gravity is about 9.807 depending on your place on earth...

Overview of the Pneumatic parts with the calculated piston areas:

part description


 
part nr at Peeron or Bricklink
 
area (right) chamber area (left) chamber
cm² square
inch
cm² square
inch
Technic Pneumatic Cylinder New with Hole Base (Complete Assembly) 47224c01
 and
2793c01
 1.368  0.212  1.172  0.181
Technic Pneumatic Cylinder Small Complete Assembly  x189c01  0.301  0.046  0.221  0.034












Experiment I: Technic Pneumatic Cylinder (part no:47224c01 and 2793c01) pushing
In this test the cylinder is pushing the lever up on one site. On the other site of the lever the weighing device is pulling down and measuring the force in Kilogram force. At the seem time the NXT is displaying the pressure within the cylinder in mBar.















I did 12 measurements with different pressure levels and wrote down the Kilogram force displayed at the weighing device.
In the chart below you can see the difference between the actual measured force and the calculated force.

The pressure measurements I have made 262 ,436, 720, 968, 1211, 1460, 1631, 1716, 1978, 2008, 2158 en 2379 in mBar.

Using these pressure measurements we can calculated the force performed by the cylinder:

[area of the piston = 0,0001368 m²]

The force is calculated as followed:
F = P (pressure in Pascal) x A (area in m²)

F = 100 x pressure in mBar x
0,0001368 =
F = 100 x 262 x
0.0001368 = 3.58 Newton
F = 100 x 436 x
0.0001368 = 5.96 Newton
F = 100 x 720 x 0.0001368 = 9.85 Newton
etc...

The weight measurements I have made: 0.72, 1.02, 1.62, 2.06, 2.56, 3.06, 3.30, 3.62, 4.04, 4.08, 4.28 and 4.90 KG

Using these weight measurements we can calculated the force measured by the weighing device:

[gravity = 9.807 meters per second] [lever proportion = 6/10]

The force can be calculated as followed:
F = weight  x  6/10 lever proportion  x  gravity

F = weight  x  0.6  x  9.807 =
F = 0.72      x 0.6  x   9.807 = 4.24
Newton
F = 1.02      x 0.6  x   9.807 = 6.00 Newton
F = 1.62      x 0.6  x   9.807 = 9.53 Newton
etc..

note: The calculated force should be a straight line and the measured force should be at least nearly a straight line, but I was not able to made samples with a equal interval. Excel can't display the values on the horizontal axle correct. It displays the pressure values on the horizontal axle with a fixed interval, but as I said, my samples ain't!
Click here for the Excel sheet with the results of experiment I:































It turn out to be that the calculated force is a bit higher. But I found it remarkable that theory and practise come this close! The difference between theory and practise is probably a result of friction and tolerance in the calculation of the pistion area of the cylinder.


Experiment II: Technic Pneumatic Cylinder (part no:47224c01 and 2793c01) pulling
In this test the cylinder is pulling the lever up on one site. On the other site of the lever the weighing device is pulling down and measuring the force in Kilogram force. At the seem time the NXT is displaying the pressure within the cylinder in mBar.




















I did 12 measurements with different pressure levels and wrote down the Kilogram force displayed at the weighing device.
In the chart below you can see the difference between the actual measured force and the calculated force.

The pressure measurements I have made: 99, 165, 201, 485, 659, 783, 1068, 1145, 1360, 1454, 1838 en 2426 in mBar.

Using these pressure measurements we can calculated the force performed by the cylinder:

[area of the piston = 0,0001368 m²]

The force is calculated as followed:
F = P (pressure in Pascal) x A (area in m²)

F = 100 x pressure in mBar x
0,0001368 =
F = 100 x  99  x
0.0001368 = 1.16 Newton
F = 100 x 165 x
0.0001368 = 1.93 Newton
F = 100 x 201 x 0.0001368 = 2.36 Newton
etc...

The weight measurements I have made: 0.28, 0.42, 0.46, 0.88, 1.18, 1.38, 1.88, 2.02, 2.20, 2.46, 3.12 and 4.16 KG

Using these weight measurements we can calculated the force measured by the weighing device:

[gravity = 9.807 meters per second] [lever proportion = 6/10]

The force can be calculated as followed:
F = weight  x  6/10 lever proportion  x  gravity

F = weight  x  0.6  x  9.807 =
F = 0.28  x 0.6  x   9.807 = 1.65
Newton
F = 0.42  x 0.6  x   9.807 = 2.47 Newton
F = 0.46  x 0.6  x   9.807 = 2.71 Newton
etc..

note: The calculated force should be a straight line and the measured force should be at least nearly a straight line, but I was not able to made samples with a equal interval. Excel can't display the values on the horizontal axle correct. It displays the pressure values on the horizontal axle with a fixed interval, but as I said, my samples ain't!
Click here for the Excel sheet with the results of experiment II:

































Again it turn out to be that the calculated force is a bit higher. The difference between theory and practise is probably a result of friction and tolerance in the calculated piston area.

Experiment III: Small technic Pneumatic Cylinder In this test the cylinder is pushing the lever up on one site. On the other site of the lever the weighing device is pulling down and measuring the force in Kilogram force. At the seem time the NXT is displaying the pressure within the cylinder in mBar.























I did 8 measurements with different pressure levels and wrote down the Kilogram force displayed at the weighing device.
In the chart below you can see the difference between the actual measured force and the calculated force.

The pressure measurements I have made 500, 797, 957, 1106, 1
620, 1818, 1990 and 2318 in mBar.

Using these pressure measurements we can calculated the force performed by the cylinder:

[area of the piston = 0,0000301 m²]


The force is calculated as followed:
F = P (pressure in Pascal) x A (area in m²)

F = 100 x pressure in mBar x
0,0000301 =
F = 100 x 500 x
0.0000301 = 1.51 Newton
F = 100 x 797 x
0.0000301 = 2.41 Newton
F = 100 x 957 x 0.0000301 = 2.89 Newton
etc...

The weight measurements I have made: 0.20, 0.34, 0.44, 0.50, 0.74, 0.82, 0.88 and 1.08 KG

Using these weight measurements we can calculated the force measured by the weighing device:

[gravity = 9.807 meters per second] [lever proportion = 6/10]

The force can be calculated as followed:
F = weight  x  6/10 lever proportion  x  gravity

F = weight  x  0.6  x  9.807 =
F = 0.20 x 0.6  x   9.807 = 1.18
Newton
F = 0.34 x 0.6  x   9.807 = 2.00 Newton
F = 0.44 x 0.6  x   9.807 = 2.59 Newton
etc..

note: The calculated force should be a straight line and the measured force should be at least nearly a straight line, but I was not able to made samples with a equal interval. Excel can't display the values on the horizontal axle correct. It displays the pressure values on the horizontal axle with a fixed interval, but as I said, my samples ain't!
Click here for the Excel sheet with the results of experiment III:





























I noticed again during the measurements that  there is difference between the calculated and the measured force performed by the LEGO pneumatic cylinder.

Experiment IV: Small technic Pneumatic Cylinder In this test the cylinder is pulling the lever up on one site. On the other site of the lever the weighing device is pulling down and measuring the force in Kilogram force. At the seem time the NXT is displaying the pressure within the cylinder in mBar.


























I did 8 measurements with different pressure levels and wrote down the Kilogram force displayed at the weighing device.
In the chart below you can see the difference between the actual measured force and the calculated force.

The pressure measurements I have made 500, 797, 957, 1106, 1620, 1818, 1990 and 2318 in mBar.

Using these pressure measurements we can calculated the force performed by the cylinder:

[area of the piston = 0,0000301 m²]

The force is calculated as followed:
F = P (pressure in Pascal) x A (area in m²)

F = 100 x pressure in mBar x
0,0000301 =
F = 100 x 500 x
0.0000301 = 1.51 Newton
F = 100 x 797 x
0.0000301 = 2.41 Newton
F = 100 x 957 x 0.0000301 = 2.89 Newton
etc...

The weight measurements I have made: 0.20, 0.34, 0.44, 0.50, 0.74, 0.82, 0.88 and 1.08 KG

Using these weight measurements we can calculated the force measured by the weighing device:

[gravity = 9.807 meters per second] [lever proportion = 6/10]

The force can be calculated as followed:
F = weight  x  6/10 lever proportion  x  gravity

F = weight  x  0.6  x  9.807 =
F = 0.20 x 0.6  x   9.807 = 1.18
Newton
F = 0.34 x 0.6  x   9.807 = 2.00 Newton
F = 0.44 x 0.6  x   9.807 = 2.59 Newton
etc..

note: The calculated force should be a straight line and the measured force should be at least nearly a straight line, but I was not able to made samples with a equal interval. Excel can't display the values on the horizontal axle correct. It displays the pressure values on the horizontal axle with a fixed interval, but as I said, my samples ain't!
Click here for the Excel sheet with the results of experiment IV:





























Again it turn out to be that the calculated force is a bit higher. The difference between theory and practise is probably a result of friction and tolerance in the calculated piston area.

I have made an other page about measurements and calculations on LEGO pneumatic cylinders: Pneumatic LEGO & laws of physics I


Check the complete Pneumatic LEGO & the laws of physics II high resolution photo gallery at Google Picasa...

 
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