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Hammer Raising Example

Example of Energy Analysis in Machine Design

 Problem Definition and Overview of Analysis

 The objective of this analysis is to determine the feasibility and method for lifting the hammer shown below. The hammer is to be rotated about a pivot from an initial position to a desired final upright position as shown in the figure. The energy for raising the hammer is to be provided from a given DC brush motor. The analysis first determines how much energy and power the motor needs to raise the hammer within the specified time limit. A separate analysis will be needed to determine an appropriate gear ratio between the motor and the arm necessary to accomplish the task.

 

Design Parameters:

mass of hammer: m = 0.5kg (kilograms)

length from pivot to center of gravity of hammer: L = 0.2m (meters)

desired time for raising hammer: traise = 20 s (seconds)

factor of safety: F.S. = 2 (this is a high factor of safety, but suitable for feasibility analysis)

 

Assumptions:

  1. Hammer moves slowly. Thus dynamic forces can be neglected
  2. Friction in pivot is relatively small and can be neglected

 

Energy and Power Need Analysis

 

The objective of this analysis is to provide a rough estimate of the energy source requirements. This rough analysis will be useful for selecting what is a viable energy source for this component.

 

The energy required to lift the hammer into the upright position is equal to the change in potential energy:

 

 

The amount of energy needed from a motor must include the factor of safety:

 

 

The power required to lift the hammer within the specified time limit is given by:

 

                   

 

Conclusion

 

The energy analysis provided the means for determining the energy requirement for raising the hammer.  It was determined that a motor that is capable of producing approximately 0.196 W of power is needed to overcome the change in potential energy in the desired 20 second span.  It is important to note that meeting energy and power needs is a necessary condition, but not a sufficient condition. An actual torque analysis will be required at a later stage to determine if a gear reduction or other mechanical advantage is needed.

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