MCI produces two models of coaches that feature tag axle steering; the D4500 and the E4500. The D4500 axle is a passive system consisting of a modified front axle that steers using the caster angle of the axle while the coach is moving at slower speeds. At speeds greater than 20 mph, the axle is locked in the straight-ahead position. Section 11, Tag Axle Steering - 102DL3 / D4500 deals specifically with the DL3 / D4500 steerable tag axle.
The E4500 axle is also a modified front axle, but is controlled hydraulically. The tag axle steers whenever the steering wheel is turned, regardless of whether the coach is moving or standing still. Here's how it works.
Attached to the main pitman arm at the main steering gear box is a hydraulic cylinder. As the steering wheel is turned, this cylinder is either retracted or extended, depending on the steering wheel rotation. See Figure 1.
Attached to the tag axle is a second cylinder. This tag axle (slave) cylinder is connected hydraulically to the (master) cylinder at the steering gear box. For every inch of movement at the master cylinder, the slave cylinder moves the same amount. See Figure 2.
|Figure 1||Figure 2|
The design of the pitman arm on the bottom of the steering gearbox establishes the steering ratio between the front axle and the rear axle. This ratio is preset at 5:1. In other words, for every five degrees the front axle turns, the tag axle turns one degree. The total turning angle for the tag axle is approximately 10 degrees. See Figure 3.
There are three main hydraulic circuits, labeled L1, L2, and L3 which allow the system to operate. L1 is pressurized during left-hand turns; L2 is pressurized during right-hand turns; and L3 is the supply line for the entire system. Below is a line drawing representing the Rear Axle Steering (RAS) system in the straight ahead position. See Figure 4.
|Figure 3||Figure 4|
When the steering wheel is turned, the pitman arm on the bottom of the steering gear box not only moves the drag link connected to the front axle, but also extends or retracts the master cylinder for the RAS, again depending on the direction steered. This pressurizes either the L1 or L2 line which causes the slave cylinder to move the same amount. See Figure 5.
As the steering gearbox is rotated for a right hand turn, the master cylinder is retracted, forcing oil out of the cylinder into the L2 line. As pressure increases in this line, the slave cylinder starts to retract, steering the tag axle. When the driver returns the steering wheel to the straight ahead position, the master cylinder is extended and oil pressure on the L1 line forces the tag axle back to its central position.
Other items required for this system to operate properly are an accumulator, a manifold assembly, and two additional hydraulic lines routed from the steering gear box to the master cylinder.
The additional hydraulic lines between the steering gear box and the master cylinder are required to assist with the movement of the master cylinder during steering (Refer to the 2 yellow lines in Figure 4).
The manifold contains pressure relief valves for the L1 and L2 lines, preset to 1470 psi (100 bar). If the pressure in the L1or L2 lines exceeds this setting during steering, oil is diverted to the L3 circuit. The manifold also contains a low pressure switch which activates a tell tale light on the dash if oil pressure in the RAS system falls below 100 psi (7 bar).
The purpose of the accumulator is to maintain system pressure and minimize pressure fluctuations in system. It contains a nitrogen charged bladder pressurized to 150 psi (10 bar). As the system is pressurized, this bladder is compressed so its internal pressure matches system pressure.
The RAS system is a closed system that must be pressurized by an external pump to 220-230 psi (15-17 bar) prior to operation. A synthetic oil called "Pentosin" is the only oil recommended to be used in the system and cannot be mixed or blended with any other oil.
For more information on the RAS system, consult Section 11-E of the E4500 Maintenance Manual, where you'll find further explanation about the system, including the function of the master cylinder dead band and the self centering feature of the slave cylinder.
If you need further assistance, contact the MCI Customer Support Center at (800) 241-2947.
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