Top Thrill Dragster Redesign Concept - Part 4

III. FAILURE

Since there are more interacting parts utilized with a hydraulic launch system as opposed to a magnetic one, parts are more likely to fail.

The part that seems like it would be the most likely to fail are the cables (28, 29) that pull the pusher, shown above. These cables are responsible for transferring all of the energy produced by the 32 hydraulic motors to the train. A great deal of tension is placed on these cables as it accelerates the coaster weighing, 15,000 lbs with passengers, from zero to 120 mph in 4 seconds. The stresses on these cables will fluctuate from minimum value, while the ride is idle, to some maximum value as they pull the pusher. Hence, fatigue will eventually occur.

This has in fact been the case several times during the early operating life of the ride. The expected life of this part is unknown but it seems reasonable that the designers only planned to replace the cable once or twice a season, in order to prevent failure.

So far the cables have not been lasting that long, the first one breaking after just 22 days of operation. Because of the relatively cheap cost of the cable, as well as the strength requirements, repair of the cable is not practical, in the event of failure the part is replaced. During this first failure, the part was replaced that day and operation resumed the next day. Repair would not have been much quicker if quicker at all, and would almost certainly leave the part weaker than before it had broken anyway.

The drum (24) is a shaft supported on two ends with bearings. And is subjected to fluctuating loadings due to the tension in the ropes. Hence, it can be considered as subjected to a distributed load of different magnitude in different parts of the shaft.

The free body diagram of the drum(24) can be approximated as shown below ( ignoring the angles of the cables). A torque not shown would also be acting on the drum shaft in either direction depending on if it is launching the coaster or retrieving the pusher.

The direction of the wind for cable (45) is reversed from the wind of cables (28) and (29). This means as the tension of cables 28 and 29 is maximum the tension of cable 45 is zero. The direction of rotation of the drum also reverses, This means the drum is subjected to a fatigue load. The torque also reverses direction and fluctuates from zero to maximum likewise.

Going back to the launch cables, it is pretty obvious from their function that the main force acting on them will be tensile. The loads that they will carry will be large in order to achieve such high accelerations on such a massive object (the 15,000 lb train).

In addition to tensile forces, there are undoubtedly frictional forces. The cables that are wound around the drum experience friction with both the drum and themselves. This friction can become significant as the cables are wound tightly and move rapidly as the drum spins, possibly contributing to the repeated failure of the cables.

Two of the launch cables act while launching, while the other one acts to pull back the pusher. Since each set of cables serves only one function, and acceleration during both of these operations is uniform, there is a minimal range of loads.

The environment most certainly plays a role in the fatigue failure process. The cables that pull the pusher are located directly under the track and are exposed to the environment. Because the ride is located in Sandusky, Ohio, and operates from spring to fall, it experiences a wide range of weather conditions. Rain, humidity, and changes in temperature all add together to shorten the ideally expected life of the part. The extent to these affects, as well as the aforementioned frictional affects may have been underestimated by the designers, causing for the many unexpected failures.