WO1995008066A1

WO1995008066A1 - A crosshead for a two-stroke v-engine

Info

Publication number: WO1995008066A1
Application number: PCT/DK1994/000336
Authority: WO
Inventors: Finn Quordrup Jensen
Original assignee: MAN B&W Diesel GmbH; MAN B&W Diesel AS
Current assignee: MAN B&W Diesel GmbH; MAN B&W Diesel AS
Priority date: 1993-09-13
Filing date: 1994-09-09
Publication date: 1995-03-23
Anticipated expiration: 1996-03-13
Also published as: KR960704169A; JPH09502500A; DK172977B1; JP3212610B2; KR100310085B1; DK102193D0; DK102193A

Abstract

A crosshead for a large two-stroke V-engine has a transverse stiffening (7, 8) carrying guide planes (9) for the crosshead in pairs of neighbouring cylinders. For each cylinder pair, the connecting rods (3) are journalled side by side on a joint connecting rod pin (2) with the result that the crossheads are not positioned in the middle between the transverse stiffenings. The crosshead pin (14) is asymmetrical about the journal surface (15) which is journalled in the crosshead housing, as the journal surface is connected with the two associated crosshead shoes (16, 17) via a first pin section (18) and a second pin section (19) of different lengths. The masses of the two pin sections with associated shoes are adapted so that the centre of gravity of the pin with the shoes is located at the middle of the crosshead bearing (4).

Description

A crosshead for a two-stroke V-engine.

The invention relates to a crosshead for a large two-stroke V-engine of the crosshead type having its cylinders arranged in two rows and having a transverse stiffening carrying guide planes for crossheads in pairs of associated neighbouring cylinders, each cylinder being positioned in a respective row, wherein the two connecting rods associated with a cylinder pair are joumalled side by side on a joint connecting rod pin, and wherein the journal surface of a crosshead pin is asymmetrically positioned in the longitudinal direction of the pin so that a first pin section connecting the journal surface with one crosshead shoe is longer than a second pin section connecting the journal surface with the other crosshead shoe- Such a crosshead is known from the Applicant's Danish patent application No. 1479/92. The crosshead is a vital, heavily loaded engine component which has to be very reliable in operation. In large crosshead engines with outputs of up to 5000 k per cylinder, the crosshead has to transmit large compressive forces from the piston rod to the connecting rod, and as the oscillating swinging movement of the connecting rod in relation to the piston rod takes pace at a low speed and at a limited amplitude, it is difficult to maintain a lubricating oil film of suitable thickness between the journal surface of the crosshead bearing and the sliding surface in the surrounding crosshead housing.

The object of the invention is to obtain the largest possible degree of operational reliability for the crosshead bearing with the asymmetrical pin.

With this object in view, the crosshead according to the invention is characterized in that the mass of the first pin section with the associated crosshead shoe is reduced and/or the mass of the second pin section with associated crosshead shoe is increased so that the centre of gravity for the crosshead pin with the cross¬ head shoes is located substantially at the middle of the crosshead bearing.

With this adjustment of the masses of the pin sections with associated crosshead shoes it is possible to avoid the asymmetrical crosshead pin being influ¬ enced, at the upward and downward movements, by inertia forces which attack the pin at a distance from the journal middle and thus might provoke edge pressures in the bearing, that is, a larger load at one end of the bearing than at the other. Such edge pressures will reduce the thickness of the sensitive lubricating oil film at the most heavily loaded end of the bearing with a consequent local overload of the bearing surface, increased thermal load and greater wear. Designing the asymmetrical crosshead pin and the parts connected therewith so that their joint centre of gravity is located at the middle of the crosshead, ensures that, despite the asymmetrical design of the pin, the inertia forces act on the middle of the journal surface which consequently loads the bearing surface of the crosshead in a uniform manner and without edge pressures. In a first embodiment, the mass of the first pin section is reduced by means of a centrally extending bore which, in addition to being simple to manufacture, also involves the advantage that the material in the pin section is located at a distance from the longitudinal axis of the pin so that the pin retains most of its rigidity.

In a second embodiment, over part of its length the first pin section has a smaller diameter than the second pin section, which may be realized either by reducing the diameter of the first section or by increasing the diameter of the second section, and the latter is preferred in the cases where the pin has to have a high degree of rigidity.

In a third embodiment, the crosshead shoe on the first pin section has a smaller mass than the crosshead shoe on the second pin section. By effecting the balancing through adjustment of the masses of the crosshead shoes, the crosshead pin itself may be manufactured in the usual manner. Examples of embodiments of the crosshead according to the invention will now be explained in further detail below with reference to the very schematic drawing, in which

Fig. 1 shows a longitudinal sectional view through a cylinder unit having a crosshead according to the invention,

Fig. 2 is a sectional view along the line II-II in Fig. 1 through the crossheads in two associated neigh¬ bouring cylinders, and Figs. 3 and 4 are longitudinal sectional views through the crosshead pins of a second and a third embodiment, respectively, according to the invention.

Fig. 1 shows a crank throw 1 of a crankshaft in a large low-speed V-engine of the crosshead type, which may, for example, be a propulsion engine of a ship or a power-producing stationary engine. The crank throw has a connecting rod pin 2, on which two uniform connecting rods 3 are joumalled side by side. The connecting rods lead up to respective sets of crosshead 4, piston rod 5 and piston 6 in two associated neighbouring cylinders positioned between two transverse walls 7 and 8 forming a transverse stiffening in the engine frame box located between the bedplate and the cylinder section.

The transverse walls 7, 8 are plane and through- going in the transverse direction of the frame box so as to impart great rigidity to it. The transverse walls may be single-layer stiffened plate walls or may be frame sections constructed from profiles and plate portions. Guide planes 9 for the crossheads are fastened to the transverse walls, for example by means of welding and a suitable number of stiffeners 10 on the backs of the guide planes. The guide planes extend substantially over the full height of the engine frame box and are oblique in relation to the longitudinal median plane of the frame box so that they extend in parallel with the longitudinal axis of the associated cylinder liner 11. The longitudinal axes of the two cylinder liners in a pair of neighbouring cylinders form an angle with the apex positioned at a level with or below the longitudi- nal axis of the crankshaft.

As the transverse walls 7, 8 extend mutually in parallel and at right angles to the longitudinal axis of the engine, the distance between the walls is substantially constant, and as the connecting rods are positioned side by side on the connecting rod pin, and the two crossheads in a pair of neighbouring cylinders are positioned between the same two transverse walls, the crosshead is closer to one than the other transverse wall. The crosshead 4 comprises a two-piece bearing housing, the underpart 12 of which merges with the connecting rod, and the upper part 13 of which has an upwardly open cut, through which the piston rod passes for fastening on a crosshead pin 14, which is joumalled in the bearing housing with a joumal section 15. The crosshead pin carries two crosshead shoes 16, 17, which slide along the guide planes and guide the crosshead in the transverse direction of the engine.

The crosshead pin 14 has a first pin section 18 which extends from one end of the journal section to one end of the pin, where one shoe 16 is joumalled on the pin, and a second pin section 19 which extends from the other end of the journal section to the other end of the pin, where the second shoe 17 is joumalled on the pin. As a consequence of the location of the crosshead nearest one transverse wall, the first section 18 is longer than the second section 19, so that the journal surface or section has an asymmetrical location in the longitudinal direction of the pin. In the first embodiment shown in Figs. 1 and 2, the first pin section 18 has a centrally extending bore 20, which opens out in the pin end, and the diameter and length of which are adapted so that the centre of gravity 21 of the pin is located at the middle of the joumal surface. In the following description of alternative embodiments, the same reference numerals will be used as above for details of the same type.

In the second embodiment shown in Fig. 3, over part 22 of its length, the first pin section 18 has been turned down to a smaller diameter, where the depth and position of the mass-reducing turning have been adapted so that the mass centre of gravity 21 of the pin is located at the middle of the journal surface. It is, of course, also possible to manufacture part of the second pin section 19 with a larger diameter.

In the third embodiment shown in Fig. 4, the first and the second pin sections are designed with identical diameters, but the crosshead shoe 16 on the first section 18 has a smaller mass than the shoe 17 on the second section 19, so that the centre of gravity 21' of the pin with associated crosshead shoes is located at the middle of the joumal surface. The increased mass of the second shoe 17 may be provided by increasing the diameter of the shoe, as shown, but this has the result that the associated guide planes have to be spaced further apart. As it may be advantageous, production- wise, for all associated guide planes in the engine frame box to be equidistant, the mass increase of the second shoe may alternatively be provided by mounting one or more separate weights on the shoe, for example by fastening between the sliding surface carrying arms in positions above and/or below the central journalling section on the shoe.

When the lengths of the first and second pin sections 18, 19 of a concrete engine have been deter¬ mined from the distance between the transverse walls 7, 8 and the length and position of the joumal section 15, the centre of gravity of the crosshead pin and the members carried thereby may be determined, and by the aid of quite ordinary mass moment calculations the dimensions and the location of the mass-increasing or mass-reducing means according to the invention may be determined so that the centre of gravity of the crosshead pin with associated members is made to be located at the middle of the crosshead bearing.

Obviously the above embodiments may be combined to obtain the desired location of the centre of gravity. If the crosshead pin is provided with oil distribution channels for lubrication of the guide planes, the diameter of the bore in the first pin section may be increased by as much as the oil distribution allows, and any remaining imbalance may be removed by a relatively limited increase of the mass of the second pin section and/or the second crosshead shoe.

Claims

P A T E N T C L A I M S

1. A crosshead (4) for a large two-stroke V-engine of the crosshead type having its cylinders arranged in two rows and having a transverse stiffening (7, 8) carrying guide planes (9) for crossheads in pairs of associated neighbouring cylinders, each cylinder being positioned in a respective row, wherein the two connecting rods (3) associated with a cylinder pair are joumalled side by side on a joint connecting rod pin (2), and wherein the joumal surface (15) of a crosshead pin (14) is asymmetrically positioned in the longitudinal direction of the pin so that a first pin section (18) connecting the journal surface with one crosshead shoe (16) is longer than a second pin section (19) connecting the joumal surface with the other crosshead shoe (17), c h a r a c t e r i z e d in that the mass of the first pin section (18) with the associated crosshead shoe is reduced and/or the mass of the second pin section ( 19) with associated crosshead shoe is increased so that the centre of gravity (21; 21') for the crosshead pin (14) with the crosshead shoes (16, 17) is located substan¬ tially at the middle of the crosshead bearing.

2. A crosshead (4) according to claim 1, c h a r¬ a c t e r i z e d in that the first pin section (18) has a centrally extending bore (20) which reduces the mass of the section in relation to the second pin section (19).

3. A crosshead (4) according to claim 1, c h a r¬ a c t e r i z e d in that over part (22) of its length, the first pin section (18) has a smaller diameter than the second pin section.

4. A crosshead (4) according to claim 1, c h a r¬ a c t e r i z e d in that the crosshead shoe (16) on the first pin section has a smaller mass than the crosshead shoe on the second pin section (17).