Integral Shaft Support System Targets Excess Crankshaft Loads

Diesel Progress North American Edition, July, 2000

The current trend in diesel engines is toward gaining more power and speed from the same size package. This trend also requires the engine crankshaft to accept higher and higher loads without an increase in its physical size. The result is a crankshaft and bearing system that is more sensitive to the destructive force of outside loads being applied.

One setup, common to both marine and industrial applications, consists of an engine driving equipment through a universal joint shaft. In powertrains where a universal joint is directly coupled to the engine flywheel, additional and significant static and dynamic loads are imposed on the crankshaft and bearings. To complicate things even more, most of this equipment requires a torsionally resilient coupling on the engine flywheel to isolate torque pulses from the rest of the driveline. At the same time, a conventional coupling in this type of application acts only as a torsional isolator -- the axial and radial forces are still supported by the engine crank bearings.

Traditionally, the load on the crankshaft bearings has not been a major concern because the safety factor built into engine bearings has normally been quite high. However, as more applications require greater power density in the engine, the amount of load that can be safely supported by the bearings becomes a critical factor. Several engine manufacturers have begun placing limits on the additional loads that can safely be supported by their crankshafts. The challenge then becomes finding a way to effectively isolate the crankshaft from outside loads while still being able to incorporate a torsionally resilient coupling.

Through its work with engine manufacturers, American Vulkan, Winter Haven, Fla., has developed a solution to the problem of excess crankshaft loading. The Vulkan Integral Shaft Support family of couplings incorporate a housing and bearing assembly to be used in conjunction with the company's flywheel-mounted couplings. The torsional coupling attaches to the flywheel in the usual manner and the Integral Shaft Support housing bolts directly to the engine bell housing. With this arrangement, outside loads are transmitted through the ISS housing to the engine block rather than the crankshaft, according to the company. The Vulkan torsional coupling on the flywheel dampens torsional vibration by isolating the torque pulses from the driveline.

The Integral Shaft Support system was developed after close study of the causes of additional crankshaft loads. In the most common type of installation, which is an engine connected to a universal shaft with axial length compensation via a slip spine, the additional load added to the crank has three components:

* The static weight component. This is understood as the weight of the knuckle and a portion of the shaft tube, along with the weight of a torsional coupling or stub shaft arrangement. The static load can have both a radial and axial component depending upon the orientation of the engine (i.e., whether it is level or at an angle).

* Dynamic radial forces generated by the action of the universal shaft knuckles. These dynamic loads are cyclical and the maximum load occurs twice per revolution. These loads are well understood and the magnitude can be calculated by knowing the geometry of the drive shaft and bearing system and the torque transmitted.

* Axial reaction or thrust transmitted through the shaft, spline and into the engine crankshaft. This results from outside forces such as thermal growth or flexure of the mounting structure that tend to compress or stretch the shaft. The thrust is limited to the amount of force it takes to overcome friction in the cardan shaft splines proportional to the torque transmitted and also depends upon the geometry of the drive shaft installation and the spline dimensions for the particular shaft. Any amount of thrust less than the required thrust to overcome the friction in the spline will be transmitted along the length of the shaft and into the engine crankshaft thrust bearing.

Vulkan's ISS system is designed to be used with the company's Torflex, Vulkardan-E (VKE) and Vulastik-L (VL) flywheel couplings, which are normally used in close-coupled gearboxes. The output flange of the ISS housing is machined for common universal joint interfaces.

MAN Marine has approved the Vulkan Integral Shaft Support with Vulastik-L couplings. "The advantage of this design is the ability to take up the radial and axial forces caused by the vibration, acceleration and the weight of the coupling housing," said Wolfram Heinecke of MAN Nutzfahrzeuge AG. "The coupling, flywheel and crankshaft are protected from these forces."

Ensco Marine Co. is presently undertaking major modifications to three of its 180 ft. offshore supply vessels at Eastern Shipbuilding Group in Panama City, Fla. The modifications include a total re-power incorporating new Caterpillar 3512B that drive Ulstein Z-drive propulsion units through Vulkan Vulastik-L ISS 40 couplings. The vessels will also be lengthened by 50 ft. with the addition of four liquid mud and fuel wing tanks to a new total length of 230 ft. and will be fully SOLAS compliant and equipped with new electronics including a Simrad Dynamic Positioning System. Once completed, the vessels will work from Gulf of Mexico ports to support offshore drilling operations.