couplings & dampers
The role of dampers on two-stroke engines
Torsional vibration is a concern in power transmissions using rotating shafts and couplings. How do engine manufacturers tackle the problem?
by Robert van Kleef T to
he future of two-stroke engines appears to
be heading toward larger bore
diameters and fewer cylinders. This will effectively result in shorter crankshafts and longer propeller shafts. What does this imply for torsional vibration?
Engine manufacturers have different options
avoid dangerously high vibrations. The use of a damper depends on various factors, including number of cylinders (10 and more cylinders normally need a damper to protect the crankshaft, smaller engines need a damper to protect the propeller shaft), and propulsion layout (propeller type and size, shaft material, length and geometry). Torsional dampers work on different principles. One is the friction/viscous damper. The damper is filled with a silicone-like fluid named polydimethylsiloxane (PDMS). This is a group of polymers containing a silicon-oxygen skeleton with a high viscosity (starting from 60,000 cSt) and a high viscosity index. An interesting advantage of this system is that when the vibration frequency rises, the elastic properties of the PDMS change. At higher vibration frequencies, the hydrodynamic ‘connection’ between hub and housing will create higher torsional stiffness. Holset manufactures and services viscous dampers ranging from a diameter of 965mm
A Geislinger damper set for a Wärtsilä two-stroke engine
to 3,200mm, and weighing up to 2.4 tons. Each damper is custom made but, as precision is so important, all the vibration forces and crankshaft stress levels are measured and calculated
beforehand. Geislinger markets viscous dampers for propulsion systems with smaller engines which do not have pressurised oil supply through the crankshaft (four-stroke) or for engines with no possibility for external oil supply (smaller two-stroke engines and installations with extreme space limitations or ‘power take off’ solutions). For many years Wärtsilä has been installing torsional dampers in its camshafts for its 38 and 46 type engines. The disadvantage of a viscous damper on 38 and 46 type engines in the Wärtsilä
Briefing: torsion and torsional vibration
Torsion is the twisting of an object due to an applied torque. Torsional vibration is always a concern in power transmission systems using rotating shafts or couplings. The rotational momentum of the prime mover is continuously changing in proportion to the propeller shaft. During one rotation of the crankshaft the
rotational momentum is not constant. The crank pins (journals) are not equally spaced
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and the mass of all the shaft parts are forming a torsion-elastic system. In addition, engines with a high number of cylinders can have very flexible crankshafts due to their length. The cylinder pressure due to combustion is not constant through the combustion cycle. When resonance occurs, the vibration amplitude can exceed the maximum tension the shaft can handle.
range is the damper is built into the engine itself. As the viscous damper needs to relieve its heat, it is affected by the lubrication oil temperature. With two-stroke engines, this problem does not exist, as all the viscous dampers are placed outside the engine. Wärtsilä engineer Henry van Dam says that when replacing a viscous damper on the 38 and 46 engines the cooling water pump and the connections need to be removed before removing the damper. “To avoid this, and to prevent breakdown, regular sampling of the damper fluid is essential. The colour of the fluid is an indication of the state of the damper. Another minus is the risk of leakage of the fluid although that seldom occurs.” The dynamic/spring damper
is another
solution. This is based on the principle of critical resonance damped by a primary and a secondary section, with leaf spring packs placed between them. These spring packs, together with intermediate pieces, form chambers which are filled with pressurised engine oil. When the springs bend due to torsional vibrations, the oil is pumped from one chamber to the next, causing hydro-dynamical damping. Austrian based Geislinger
is still market leader for
these kinds of dynamic dampers. Hans-Peter Broetzner, Geislinger marketing manager, says that Geislinger has delivered dampers up to
Marine Propulsion I February/March 2012 I 53
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