OVERVIEW

This series of exercises uses an engine hoist, four scales, and an engine block to allow students to predict (calculate) reactions and then measure them. This develops students’ confidence in their ability to calculate reactions and provides a great opportunity for “what if?” exercises.

PRINCIPLE

Reinforces static equilibrium and moment of a force.

We have been able to accomplish four tasks within a 55-minute lesson. This exercise is led by the instructor with student assistance. In an ideal situation, there would be several hoists available so all students could more actively participate.

1. Determine weight of engine hoist – sit the hoist on the four scales and add them up
2. Determine the location of CG for engine hoist – application of equilibrium to solve for an unknown distance rather than reactions (mind blown!)
3. Determine weight of engine block – lift the engine, record weight on four scales and add them up, difference between this total and the weight of the hoist is the weight of the block
4. Predict what happens to reactions if arm length increases
   • Build the FBD together and solve the equations of equilibrium together
   • Change the hoist and lift the engine – compare measurement to calculation

WHAT YOU NEED

• Engine hoist (we used a Strongway 2-Ton Hydraulic Engine Hoist)
• Engine block
• Load leveler (to connect to engine block; we used the one that was included with the engine hoist)
• Four bathroom scales (we found best success with spring scales; digital scales automatically turn off at the worst possible times)
• Small wooden blocks (to distribute the weight from the caster to the scale more evenly)
• Index cards (to level the hoist)

HOW IT’S DONE

1. Place a scale under each of the four casters (only the four outermost casters; most hoists have two additional casters near the middle for use when the hoist is in storage mode). To limit damage to scales and distribute the load more evenly on each scale, place a wood block between the casters and the scales. See Figure 2
2. Lift the arm to be level – cadets can use a smart phone app to check that it is level. See Figure 3.
3. The floors are most likely not level. Slide stacks of 3×5 cards between casters and wood blocks to level the hoist. Scales on the back casters should read similar values; scales on the front casters should read similar values. This may take a little trial-and-error to get the hoist leveled on the scales. See Figure 4.
4. With the hoist leveled on four scales, record the value on each scale – these are the reaction forces for the self-weight.
5. With these measured reactions and the hoist self-weight, draw a free body diagram and apply static equilibrium to determine the location of the hoist’s center of gravity.
6. Ask students to predict (using intuition) where most of the weight of the engine will be when it’s lifted: front or back casters.
7. Lower the arm, attach the engine, and hoist the engine. Lift the engine until the arm is level again. See Figure 5.
8. Record the value on each scale – these are the reaction forces including the engine weight.
9. The difference between the reaction forces including the engine weight and those for the self-weight of the hoist sum to be the weight of the engine.
10. Lower the engine block back to the ground
11. Predict what would happen to the reactions if the hoist arm were extended. Extend the arm to the desired length using the preset locations. Be sure to properly secure the arm with the bolt. Use static equilibrium to calculate what the reaction should be and then lift the engine to check by reading the values on the scales.
12. Calculate where the engine would have to be positioned to cause the hoist to tip (zero reaction on the rear casters). Discuss why the hoist arm does not extend out that far.
13. There are likely other “what if?” scenarios that can be addressed with this physical set-up.

REFERENCES

For more about this exercise and the principles the authors used to develop it, please see this paper.

For more about a refit of the statics course in which this exercise was developed, please see this paper.