Representatives of a local municipal garage parking operatio

Representatives of a local municipal garage parking operation are considering installing hydraulic lifts to raise automobiles from one level of their building to another. They feel as though this will reduce the vandalism that has lately occurred through the easy ramp access to each level. The racks and hydraulic cylinders necessary have been designed and specified. Each rack will weigh 2,450 pounds and will be raised by a 3 inch diameter ram having a 68 inch stroke. Building codes limit the maximum weight of automobiles allowed in this garage to 4,500 pounds thus, the total load possible will be 6,950 pounds on extension of the cylinder. The representatives feel that a great savings in cost can be achieved if two or three portable hydraulic power supplies could be used to operate these lifts rather than having a built-in power supply dedicated to each lift. They are seeking, at this time, your consultation through supplying design and specifications for a portable power unit that will operate these lifts to fully raise each automobile in 20 seconds. Your report to them should include sizes, ratings, designs, types, etc., of circuitry and components. Therefore, you will need to supply the information required in the following directions. It is important to show your calculation work for each problem. • Calculate the maximum pressure that will be imposed upon the power supply during extension of the cylinder. • Calculate the delivery required to fully extend the cylinder in 20 seconds. • Determine the setting for the pressure relief valve used in the power supply. • What type of pump will be used in the power supply and what volumetric efficiency will be designed for? Explain your choice. • Calculate the horsepower rating for the engine (drive motor) at the maximum pressure, delivery and volumetric efficiency for the power supply. If the motor in the power supply operates at a drive speed of 700 RPM what will the necessary displacement for the pump be in consideration of volumetric inefficiency? • Determine the range of volume of fluid that should be contained in the reservoir. • Describe the rating range for the strainer located in the pump inlet line. • Describe the permissible range of inside diameter sizes for conductors in the power supply (decimal size). • Select nominal (fractional) sizes of schedule 40 piping to be used in the power supply.

Solution

Total mass = 6950 pounds = 3152.5 Kg

Ram diameter = 3 inch = 0.0762m

Ram stroke = 68 inch = 1.727m

Total work done = 3152.5 x 9.81 x 7.727 = 53409.25 J = 53.4KJ

Power required = Work done / Time = 53409.25/20 = 2670.4W = 2.67KW

Peak Pressure developed = Force / Area = (3152.5 x 9.81) / (/4 x 0.0762^2) = 6.78MPa

Flow velocity = Power/Force = 2670.4/(3152.5 x 9.81) = 0.0864m/s

Discharge = Area x Flow Velocity = /4 x 0.0762^2 x 0.0864 = 0.000394015 m^3/s = 394.015 cm^3/s

Considering volumetric eff.,

Total power required = Theoretical power / Vol. efficiency = 2670.4/0.92 = 2902.6

External gear pump shall be used, since it has considerably lesser weight. And the volumetric efficiency shall be taken as 92%.

Volumetric efficiency tends to decrease with increase in size and discharge. Hence three portable hydraulic units is recommended to improve the volumetric efficiency.

Power of single motor = 2902.6/3 = 967.53W

(Max. four sub division are answerable per question as per chegg guidelines)