A bolster or turntable is a device (or rather a set of devices) that allows long loads to be transported on two transporters, with a relatively short length, instead of one single transporter with a length (almost) equal to the load. Such a transport is also referred to as a “dolly transport”. The turntables or bolsters allow transporters to pivot underneath the load to negotiate turns and corners and (to a certain extent) super elevation and articulation, as well as inclines and declines.
A turntable consists of two parts: a lower part that is secured to the transporter called the Lower Fixed Part (LFP) and a movable or rotating upper part that is secured to the load, called the Upper Rotating Part (URP). The URP rotates around the center of the whole assembly. The two parts are held in place by a pin or a ball bearing. The contact area between the URP and the LFP is lubricated with grease or oil to allow the rotating motion with minimal friction.
There are three basic turntable designs:
- The simplest design consists of just two steel plates with a pin in the center. No articulation in either direction is possible (except what the plywood between the saddle and plate allows, crushing). A-type.
- Turntables with sliding shoes and a center pin. The articulation in longitudinal direction of the load is in the shoes, it allows the URP of the turntable to pivot. B-type.
- Turntables with sliding shoes and a (load carrying) ball bearing. On this type of turntables it is sometimes possible to remove the sliding shoes (on one of the turntables) and have the load suspended solely on the ball bearing. C-type.
Figure 2 illustrates the difference between A-type and B-type turntable. The pivoting sliding shoes and the rotating pin are visible between the LFP and the URP. Even though it appears that the center pin may carry some load, this is not the case. The URP pivots on the center pin, which is allowed a free vertical movement (to a certain extent) in the hole of the LFP.
The turntable in Figure 3 is a C-type turntable. In the center of the turntable the ball bearing can be indicated. This ball bearing allows articulation in all directions, which makes the C-type the most versatile of turntables.
There is however a principle unpredictability in the C-type turntable, for the one that has both shoes in contact with the sliding ring. The loads on the shoes and center ball bearing are statically undetermined and loads can shift (about the ball bearing) quickly from one shoe to another. This can be nerve racking to the "not so experienced" operator. The center pin of the B-type turntable is not load bearing and therefore does not have this problem.
There is a common disadvantage of the B-type and C-type turntables. During transport, both operators (front and rear transporter) are aiming to maintain their transporter level (trying to keep their pressures equalized) and could herewith be “fighting” the other operator on uneven roads or underground. Such transports should be carried out by operators that know each other in terms of how they will act and react.
The C-type turntables (since the ball bearing is load carrying) can have the sliding shoes removed of one of the turntables. This eliminates the “fighting each other” scenario, as the transporter, where the sliding shoes are removed, can now freely articulate underneath the load. The transporter, that does not have the sliding shoes removed, is to maintain the level of the load. Figure 4 shows a transport where the sliding shoes have been removed from the rear turntable. Although turntables are available in various models they all follow one of the above design concepts.
When using two turntables, both with two shoes in contact with the sliding ring, one has effectively created a 4-point suspension scenario. As known, a 4-point suspension is prone to overloading on one point. The same applies to turntables.
To avoid this overloading, a 3-point suspension turntable was invented. This consisted of one turntable with two shoes in contact with the sliding ring and one turntable with just the center ball bearing. In practice, however, this 3-point suspension method is not being used as much as the 4-point suspension method.
When using the 4-point suspension method (sliding shoes on both turntables in place), it is common to use turntables that have an overcapacity and the overloading is not, or less of a concern, during execution of the transport. The hydraulic pressures of both transporters have to be continuously monitored and the operators should have contact with each other. As a dolly transport is normally carried out with low velocity, there is ample time for corrections.
Another aspect of attention is the loading impact of the turntable on the transporter. By definition, the turntable is imposing a point load (a concentrated load) onto the transporter deck and this needs to be within the capacity of the transporter. In case the point load is too concentrated, load spreaders can be used to lengthen the loading area. Figure 5 shows a turntable on a double set of load spreaders; in this case the transporter was subject to a concentrated point load that was beyond the capacity of the transporter. Figure 6 is the transport where the turntables from Figure 3 and Figure 5 are used. The front transporter required additional load spreading that can seen on Figure 6. Between the 3rd and 4th axle the load spreading starts and between the 9th and 10th axle it stops. The rear transporter did not require any load spreading as this transporter has a higher bending moment.
When to use turntables
There are a number of instances where such a configuration is desirable or even a requirement to carry out the transport.
Long loads, irrespective of weight
Long loads, irrespective of weight, that need to be maneuvered in such a way that a structure (or structures) in the surrounding area demands the maneuverability of a dolly configuration as opposed to a single transporter. Such structures can be site restrictions, trees, light posts, buildings, roundabouts etc.
Figure 7 shows the transport of a 185 ton vessel in dolly configuration due to the restrictive infrastructure. The curve (radius) of the roundabout prohibited the use of a single longer transporter. This transport was carried out in Dubai. The dolly configuration in question was 12-axle lines Cometto pull type transporter in the front (prime mover side) and 6-axle lines Cometto pull type in the rear.
Loads that are long but not heavy
Loads that are long but not heavy compared to their length and do not need to be supported over their full length. In such cases, it would require more axle lines to carry out the transport without turntables than it would to carry out the transport with turntables. It is difficult, if not impossible, to give numerical guidelines to make such a determination; each load is to be engineered on a case by case basis as each case depends on size, weight and center of gravity.
Figure 8 shows the transport of a 225-ton vessel in dolly configuration due to its length. Note the distance between the front and rear transporter. This transport was carried out in Dubai. The dolly configuration consisted of a 10-axle line Cometto pull type transporter in the front and a 6-axle line Cometto pull type transporter in the rear.
The above two reasons for using a dolly configuration are likely the most common reasons. However, there are two more, less common reasons for using a dolly transport configuration. When there are overhead obstructions on the transport route, such as wires, overpasses, tunnels etc and the total transport height of the cargo plus transporter plus transport beams (if any) is higher than what the overhead obstruction allows for, a different transport method is required.
Such a transport method, a transport frame, is shown in Figure 9. Turntables are used on each of the transporters, and the cargo is suspended from (hangs in) the transport frame. This allows the cargo to pass under obstructions that are just a few cm higher than the cargo itself. Note that when passing under high voltage wires additional precautions have to be taken.
The last reason for using turntables is related to axle loads on a suspended surface such as a bridge. Bridge spans generally have a maximum applicable axle load as well as a maximum total weight allowed on each span. This can result in the transport combination having to spread the load a certain distance apart to comply with the bridge restrictions. A dolly configuration can offer a possible solution.
Figure 10 shows the transport of a column over a bridge. The loads between the front and rear transporter had to be spread so that the full weight was never on one bridge span. However, the column length in combination with the saddle locations did not allow for this. For that reason, the front and rear transporter travelled on different lanes. This innovative solution ensured that the loads on the bridge stayed within the limits.
Figure 11 shows another transport over a (temporary) bridge where the axles have been spread. You may notice the absence of turntables in this figure which makes this transport NOT a dolly transport. The front and rear transporter are physically connected by a spine beam. This makes the transporter act as a single transporter, not as a dolly transport. The picture illustrates the spreading of the load between the front and rear part of the transporter. Another innovative solution.
Note: In terms of load spreading on the bridge, it makes no difference if the configuration as shown in figure 9-11 is used or a dolly configuration with two individual transporters.
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