A Designer's Guide to Hydraulic Shock Absorber Selection

A shock absorber is a device which converts mechanical energy into thermal energy. The energy transformation occurs as the shock absorber's fluid medium is forced through orifices at high velocities.

Selecting a shock absorber is not difficult if you follow the equations in this Guide. To insure adequate sizing, all inputs to the shock absorber must be known or conservatively estimated. For dimensions of shock absorbers, see our on-line catalog in the Thomas Register.

THE END GOAL OF THE EQUATIONS LISTED IN THIS GUIDE IS TO OBTAIN THE ENERGY INPUT TO THE SHOCK ABSORBER, AND THE SPEED AT WHICH IT OCCURS. IF YOU HAVE ANY QUESTIONS, CONTACT OUR FACTORY FOR PROMPT ASSISTANCE. AN IMPROPERLY SIZED SHOCK ABSORBER CAN BE A SAFETY HAZARD.

Please note: if your web browser cannot properly read this 4² as 4 with a superscript exponent of 2 then you should switch to this page where the exponent is written as exp2.

UNITS AND ABBREVIATIONS:

(USE ONLY THE UNITS LISTED BELOW IN ALL EQUATIONS IN THIS GUIDE.)

KE = Kinetic energy (in-lb.)

W = Weight (lb.)

W_E = Effective impact weight (lb.)

V = Linear velocity of impact at the shock absorber (ft/sec.)

V_R = Rotational velocity of impact (radians/sec.) at the shock absorber

F = Output force from shock absorber at impact (lb.)

F_D = Drive force (lb.)

H = Vertical height (in.)

S = Shock absorber stroke (in.)

I = Moment of inertia (lb-ft-sec.²)

T = Time (sec.)

a = Acceleration (ft/sec.²)

PART I -- SOLVING FOR VELOCITY OF SIMPLE MACHINERY

1. Air Cylinder Drive

   V = 2 [Average Cylinder Velocity (ft/sec.)]
   
   

2. Hydraulic Cylinder Drive
   
   

   V = 1.5 [Average Cylinder Velocity (ft/sec.)]
   
   

3. Machines With Constant Acceleration And Known Time

   V = aT
   
   

4. Machines With An Initial Velocity (V_O, Ft/Sec.) Plus Constant Acceleration And Known Time

   V = V_O + aT
   
   

5. Machines With A Constant Acceleration And Known Distance (Ft.)

   To Gain Speed

   V² = 2 (a) (distance)

PART II -- SOLVING FOR KINETIC ENERGY

1. Simple Systems
   
   
   1. Horizontal motion

      KE = .1865 WV²
      
      

   2. Vertical motion

      KE = W (H + S)
      
      

   3. Rotary motion

      KE = 6 I V²_R
      
      

   4. Applications involving attenuation of complex inputs, such as seismic events, explosions and weapons effects are beyond the scope of this publication. Contact Taylor Devices for assistance when sizing for a complex input.

   
   
   

2. Solving For Kinetic Energy Of Overhead Cranes
   
   
   1. Because of the "sling-shot" effect of cable-hung loads and overspeed possibilities, effective impact weights, W_E, should be used.
      
      
      1. Bridge Buffer

         W_E/Buffer = 1.3 [.5 bridge weight (lb.) + trolley weight (lb.)]

         - or -

         W_E/Buffer = .5[bridge weight (lb.) + trolley weight (lb.) + lifted load (lb.)]

         Use whichever weight is greater for kinetic energy calculation.
         
         

      2. Trolley Buffer

         W_E/Buffer = 1.3 [.5 trolley weight (lb.)]

         - or -

         W_E/Buffer .5 [trolley weight (lb.) + lifted load (lb.)]

         Use whichever weight is greater for kinetic energy calculation.

      
      
      

   2. Solve for kinetic energy per buffer

      KE per buffer = .1865 W_E V²

PART III -- SOLVING FOR DRIVE FORCE AT THE SHOCK ABSORBERS

1. A.C. Motors

   _D = 1375

   (Assumes 2.5:1 stall factor)**
   
   

2. D.C. Motors

   F_D = 1925

   (Assumes 3.5:1 stall factor)**
   
   

   ** NOTE: Both A. and B. neglect gearing power losses and slippage power losses.
   
   

3. Solving For Drive Force Of Wind For Outdoor Systems With Known Sail Area In Square Feet

   F_D = .004 (square feet sail area) (wind speed in mph.)²

PART IV -- SIZE SELECTION

1. General Notes On Shock Absorber Selection
   
   
   1. Several different shock absorber sizes may be acceptable for an application. For example, for an input energy of 400,000 in-lb. into 1 buffer, sizes 4 x 10, 5 x 6, and 6 x 3 all have sufficient capacity. Size selection depends on allowable deceleration, mounting arrangement, available space and selling price.
      
      
   2. If you require more or less stroke than is shown as available in a given size shock absorber, contact our factory. Semi-standard units are available with strokes of .03 to 60 in.
      
      
   3. Inputs to the shock absorber must be accurately determined, or conservatively estimated. A shock absorber that "bottoms-out" in service because of insufficient energy capacity will force the back-up structure behind the shock to absorb the energy overage. This will often result in damage to the mounting structure, the shock, or both.
      
      
   4. If your application appears to be more complex than the cases treated here, call 716-694-0800 and ask for sizing assistance.

   
   
   

2. Selecting The Shock Absorber If Input Is Pure Kinetic Energy With No Motor Drive
   
   
   1. For Taylor M-Series Fluidicshoks, H-Series Fluidicshoks, and Crane and Industrial Buffers, energy capacities are listed in the catalog tables.

      Select a shock absorber with adequate energy capacity for your calculated input. For cyclic rates above 240/hour, use a 30% safety factor on energy capacity. For cyclic rates above 600/hour, consult factory on your application.
      
      

   2. For Taylor W-Series products and Uni-Shoks, sizing grids are provided in the catalog. Sizing information for our W-Series and Uni-Shok product lines is provided in Part V of this booklet.

   
   
   

3. Deceleration Rate For Your Size Selection

   Number of g's =

   For most industrial applications, decelerations of under 8.0 g are recommended to prevent damage to electronics and to keep impact noise down. For lower decelerations, use a longer stroke unit.
   
   

4. Deceleration Time For Shock To Stroke

   
   
   

5. Deceleration Rate For Overhead Cranes
   1. 
      
      
   2. AISE 1969 code limits decelerations to .5g at 50% speed, which effectively is 2.0 g at 100% speed for a Taylor Buffer. Any buffer meeting AISE 1969 automatically complies with all OSHA regulations.
      
      
   3. OSHA code limits bridge decelerations to .093 g at 20% speed, which effectively is .373 g at 40% speed for a Taylor Buffer.
      
      
   4. OSHA code limits trolley decelerations to .146 g at 33% speed.
      
      
   5. Deceleration rate for your application is:

      Number of g's =
      
      

   6. Bridge weight per buffer for deceleration calculation, use .5 bridge weight + .5 trolley weight.
      
      
   7. If your deceleration is too high, try a longer stroke.
      
      
   8. Deceleration time is listed in D. above.

   
   
   

6. Selecting The Shock Absorber If Input Is Kinetic Energy And Drive Force
   1. 
      
      
   2. Obtain kinetic energy of your input, and the motor or wind drive force.
      
      
   3. Select a trial shock absorber diameter.
      
      
   4. Solve for stroke by using the equation:

      
      
      

      The value of C, the efficiency coefficient, is .8 for Taylor Fluidicshoks, .9 for Taylor Crane Buffers. Remember to use a 30% safety factor on kinetic energy for cyclic rates above 240/hour, and consult factory for sizing of units with cyclic rates above 600/hour.

PART V -- THE W-SERIES SELF-ADJUSTING TAYLOR DEVICES' SHOCK ABSORBER

The Taylor Devices' W-Series Shock Absorbers include our Uni-Shoks; our Fluidicshok models 1 x 1 W, 1 x 2 W, 1.25 x 2 W, 1.5 x 3 W; and our Crane Buffer models 1.5 X 3 W, 2.5 x 3 W, 3 x 4 W, 4 x 6 W, 5 x 8 W, 6 x 8 W, 6 x 14 W, 7 x 10 W, 7 x 16 W, and 7 x 20 W. These products are unique in the hydraulics field because their patented Fluidic Amplifiers will adjust shock force automatically to compensate for weight, speed, and drive force variations.

When impacted, a W-Series Shock Absorber will instantaneously apply a small test force to the impacting weight. This test force is approximately 1% of the buffer's maximum possible output force. The test force is applied for a distance of 3% of the buffer's stroke. A Fluidic Amplifier inside the shock absorber senses how the application of the test force affects the impact weight, and from this data can determine what the velocity, weight, and drive force to be absorbed actually is. With this information, the Fluidic Amplifier will set the shock force at whatever value is required to absorb the energy within the stroke of the shock. The fluidic circuits necessary to accomplish self-adjustment are built into the piston head of the buffer, and consist of 3 parts, only one of which moves.

1. Selection Of A W-Series Or Uni-Shok Shock Absorber
   
   
   1. Obtain or conservatively estimate the impact weight and speed of your application.
      
      
   2. Convert any drive forces into effective impact weight and speed using the formula:

      
      
      

   3. Add the effective weight from A.2. above and the actual impact weight from A.1. above together. Then find the point on the W-Series or Uni-Shok capacity diagrams that corresponds to this total weight. If your total weight falls off the graph, spread the weight over more than one shock until you are on the graph. if your speed is off the graph, consult factory on size required
      
      
   4. Find the point on the capacity diagram which corresponds to the total effective weight from A.3. above and the impact velocity. The point will fall in the region of one of the W-Series or Uni-Shok Shock Absorbers. This is your correct size. If your velocity is outside the range of the W-Series or Uni-Shok Shock Absorbers, you must go to a Taylor Devices' custom orificed product.

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