Skip to main content

Crosshead Lubrication


The crosshead on a slow speed 2 stroke is a difficult bearing to lubricate effectively. The load is continually downward and because the con rod swings about the pin, changing direction each stroke, true hydrodynamic lubrication cannot take place. Instead the lubrication starts as boundary, and as the rubbing speed increases, a hydrodynamic film is built up. As the rubbing speed decreases the lubrication becomes boundary once again.

 As engine powers and thus gas loads have increased, the difficulties with achieving effective lubrication have increased. Larger pin diameters have helped by increasing the linear rubbing speeds, and the continuous lower bearing has reduced the loading/unit area.
 The older forked type crosshead as found in earlier engines (up to the mid 1980s) used various methods to improve the lubrication of the crosshead. Oil grooves in the lower bearings were used to distribute the oil. The grooves in some cases extended to the edge of the bearing, although with a reduced csa, to ensure a flow of oil through the bearing.

THE MAN B&W MC ENGINE CROSSHEAD


 The lower half of the bearing housing is formed by the top end of the connecting rod. It supports the crosshead pin over its entire length, the piston rod being bolted to the top half of the crosshead pin through a cut out in the bearing top half. Oil supply to the crosshead is via a telescopic pipe from the main LO supply at a pressure of about 2.5 bar.

 The lower  bearing shell (tin aluminium with overlay) has oil grooves with machined wedges as shown in the diagram and photo. The oil enters via the cut out channel in the centre. The grooves extend right to the edges of the bearing to ensure a flow of oil, thus cooling the bearing.

 THE SULZER RTA ENGINE CROSSHEAD

  The early RTA had a forked crosshead with the piston rod passing through a hole in the crosshead pin and secured underneath with a nut. Oil entered the bearing through holes in the shell. via a groove machined in the the lower bearing housing

 

 OIL SUPPLY PRESSURE.

 A Question sometimes asked is why do Sulzer need to boost their crosshead oil supply pressure to 12 bar whilst MAN B&W supply oil to their crosshead at system pressure. The answer lies in the design of the bearing.

More than 90% of the circulated oil has the sole purpose of cooling the bearings. If you study antique machines with open crankcases, you will see that the amount of oil for lubrication is a few drops per minute. This is enough for maintaining the oil film in the bearing and with an open crankcase the friction heat is removed by air-cooling. Modern engines have closed crankcases and a much higher bearing load - hence the need for oil cooling.

In a main bearing, the oil is pumped into the upper shell and it will cool the upper part of the joumal. Since the shaft is rotating, it is cooled on all sides and because the oil film thickness is very small in the loaded part, the shaft will cool the loaded bearing half as well.

A crosshead bearing is only oscillating and the lower shell is always loaded. The cooling oil must be injected between shaft (crosshead pin) and lower bearing.

In MAN B&W engines, a set of channels have been machined in the lower crosshead bearing, in which the cooling oil can pass. The geometry is designed in such a way that all the loaded square centimetres of the pin are flushed with cooling oil twice every engine cycle. In contrast, the Sulzer crosshead has a plain lower bearing without channels. In order to inject oil between pin and bearing, they have to supply oil at a much higher pressure. The injection will take place at around 20 degrees crank angle before TDC, where the cylinder pressure is still low and upward inertia forces on piston is still high. There is a short interval, in which the bearing pressure is lower than the oil pressure.

 

Comments

  1. Another fantastic article stolen from Rollo Tomkins by another wobble bastard

    ReplyDelete
    Replies
    1. Shut the fuck up you racist fuck

      Delete
    2. Your mom wobbles when i give her the d

      Delete
    3. its for study reference you dumb, not everyone has time to read all books

      Delete

Post a Comment

If you have any doubts.Please let me know

Popular posts from this blog

Differences between MC/MC-C and ME/ME-C engines

The electrohydraulic control mechanisms of the ME engine replace the following components of the conventional MC engine: Chain drive for camshaft Camshaft with fuel cams, exhaust cams and indicator cams Fuel pump actuating gear, including roller guides and reversing mechanism Conventional fuel pressure booster and VIT system Exhaust valve actuating gear and roller guides Engine driven starting air distributor Electronic governor with actuator Regulating shaft Engine side control console Mechanical cylinder lubricators. The Engine Control System of the ME engine comprises: Control units Hydraulic power supply unit Hydraulic cylinder units, including: Electronically controlled fuel injection, and Electronically controlled exhaust valve activation Electronically controlled starting air valves Electronically controlled auxiliary blowers Integrated electronic governor functions Tacho system Electronically controlled Alpha lubricators

Main engine interlocks

Interlocks are provided so that the engine can be started or reversed only when certain conditions have been fulfilled. When there is a remote control of engines, it is essential to have interlocks. This reduces the possibility of engine damage and any hazards to the operating personnel. Turning gear Interlock . This device prevents the engine from being started if the Turning gear is engaged. Running Direction Interlock . This prevents the fuel from being supplied if the running direction of the engine does not match the Telegraph. Starting Air Distributor in end position . This prevents starting from taking place if the shifting of the Distributor has not been completed. Main Lube. oil pressure, Piston cooling pressure, Jacket water pressure, and important parameters must be above the required minimum. Auxiliary Blower Interlock . The Auxiliary Blower is provided in case of Constant pressure turbo charging. Air Spring pressure Interlock . In case of the present generation

Why is a man hole door elliptical in shape?

Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole  is lesser in size than the corresponding circular hole. More over it is prime concern to have a  smoothed generous radius at the corners to eliminate stress concentration. Hence other  geometrical shapes like rectangle and square are ruled out.  To compensate for the loss of material in the shell due to opening, a doubler ring has to be  provided around the opening. The thickness of the ring depends on the axis length along the  dirrection in which the stresses are maximum and the thickness of the shell. It is important to  align the minor axis along the length of the vessel, as the stress in this direction is  maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t=  thickness of the shell plating  Circumferential stress: Pd/4t  More over a considerable material and weight saving is achieved as minor is along the  direction of maximum stress.