Fluid Power Journal

Problem: Vehicle Rod Sizing

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In designing a vehicle to go down the road at 60 mph, one must know the drawbar pull of the vehicle. This is a combination of drag coefficient, rolling resistance, grade of incline, and any mechanical inefficiency. To find this information without a wind tunnel, one might use a scale of some type and pull the vehicle with a long tow-rope. Using a pneumatic cylinder with the blind end open to atmosphere and the rod end closed and fitted with a pressure gauge, we can make a nice scale that can be used to determine the horsepower. If the pneumatic cylinder has a bore of 3 inches, what would be the rod size required to have the pressure on the gauge be equal to the drag horsepower when traveling at 60 mph?

Find Out The Solution
Equation to find HP: HP =

pounds of full x (feet / second)

Let’s use 10 hp as an example.

(5,280 feet in a mile / 60 seconds in a minute) = 88′ / sec.

Solving: 10 x 550 / 88 = 62.5 pounds of pull. This means it takes 10 hp at 88’ / sec. with 62.5 # of pull.

Using F = PA: We will use 10 psi because that equals our 10 hp.

Force of 62.5 / 10 psi will need 6.25 sq. in. of rod end area (EREA).

The area of a 3″ cylinder = 7.068 sq. in.

Cap end area – rod end area (EREA) = rod area.

7.068 – 6.25 = 0.818 sq. in. of rod area.

Solve for diameter           A = D2 x 0.7854

Working backwards:      A / 0.7854 = D2

√ of (0.818 / 0.7854) = 1.02″ or a 1″ rod. Now your pressure gauge will read both PSI and HP.

Deadline past. Not available for submissions.


George Fling, CFPS, Southwestern Controls, Inc., Dallas, TX

Answered Correctly:

Karl Kersker, CFPS; CFPE, ATK Launch Systems, Pleasant View, UT

Kevin Peyton

Ehren Polheber, CFPS, Scot Forge Company, Salem, WI

Timothy Preston, CFPS, Berendsen Fluid Power Ltd, Smyrna, GA

Jim Lane, Bosch Rexroth

By Ernie Parker, AI, AJPP, AJPPCC, S, MT, MM, MIH, MIP, MMH,
Fluid Power Instructor, Hennepin Technical College, EParker@Hennepintech.edu

This teaser is printed in the Fluid Power Journal. Those who submit the correct answer before the deadline will have their names printed in the Society Page newsletter and in Fluid Power Journal. The winners will also be entered into a drawing for a special gift.

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