Details About ""BEDPLATE""

Image Link

Simply:  a bed plate or frame, usually of steel or cast iron, upon which an engine or other machine rests and to which it is attached.

Briefly: The bed plate acts as the main strength member, maintains correct alignment and supports the weight of the components. It must be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure.

The bed plate acts as the main strength member, maintains correct alignment and supports the weight of the components. It must be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure. It must be rigid enough to support the weight of the rest of the engine, and maintain the crankshaft, which sits in the bearing housings in the transverse girders, in alignment. At the same time it must be flexible enough to hog and sag with the foundation plate to which it is attached and which forms part of the ships structure. (If the bed plate was too rigid, then as the hull flexed, the holding down bolts, which secure the engine into the ship would be likely to break, and there would be a danger of the bed plate cracking.)

Construction: Basically the bed plate consists of two longitudinal girders which run the length of the engine. Connecting these longitudinal girders are the transverse girders which are positioned  between each crankshaft throw, and either side of the thrust collar. Built into the transverse girders are the main bearing pockets for the crankshaft to run in.

It Consists of the following;-

Longitudinal Girder (Fabricated) – the longitudinal strength is obtained by making each side of the bed plate in a form of a girder

Transverse (Cross) Girder – in which the main bearing are housed gives the bed plates its transverse strength

Longitudinal girder

Longitudinal girder apart from giving longitudinal strength, maintains alignment by giving sufficient rigidity (inflexible) to withstand the hogging and sagging of the ship’s hull.(single and double)

Ø  It provides a stiff support for the attachment of transverse girders (double)

Ø  Longitudinal girders may be of single or double plate construction

Ø  The double plate form is more stiff but would not be simple as single plate construction

Ø  The access to the holding down bolts will more difficult.

Ø  The use of single plate form is more popular with modern engines employed box bedplates and similar construction columns

Transverse Or Cross Girder

Consists of deep plate between the longitudinal girders, connected to the same on both sides.

Transverse girder carries main bearing pocket.

Ø  Deep plate is necessary for sufficient stiffness and to withstand the firing load without bending

Ø  Inadequate stiffness will distort (bend) the bearing pocket which will ‘nip’ the main bearing. This would ultimately grip the crankshaft journal and cause white metal to ‘wipe’.

Ø  These girders also may be of single or double plate form

Ø  The double plate arrangement gives high strength and stiffness.

Ø  Hole have to be cut in the plate to allow access for welding and inspection. These hole can seriously weaken a double plate arrangement in a small engine

Ø  A tube may be welded through the girder hole to restore strength and stiffness

Ø  The attachment of transverse girders to the longitudinal girders may differ (depending upon the material used).In most cases, it is welded

Ø  Some cases it may be bolted if the girder is cast, this reduces repair difficulties, allows stress relieving of the transverse girder only (classification society requirement). This also reduces risk of distortions

Modern Bed Plate: The modern bed plate is constructed from fabricated longitudinal girders with cast steel transverse sections containing the bearing pockets and tie bolt holes welded into place. After manufacture, the bed plate is stress relieved, the bearing pockets are line bored and landing surfaces machined. 

Fundamental requirement

Bedplate should provide an oil tight chamber, housing for the thrust bearing. Apart from that

Bedplate should fulfill the following:-

1          Strength

Ø  Necessary since considerable forces can be exerted.

Ø  In other word, withstand forces without failure.

Ø  These may be due to out of balance effects, vibrations, gas force transmission and gravitational forces.

2          Rigidity

Ø  Required to give correct alignment to running gear.

Ø  However, a certain degree of flexibility will prevent high stress that could be caused by slight misalignment (crack)

3          Lightness

Ø  Important, it may enable the power weight ratio to be increased.

Ø  In other word, to increase power weight ratio or power/weight (power more bigger than the weight)

Ø  Less material would be used bringing about saving cost

4          Toughness

Ø  In a material is a measure of its resilience and strength.

Ø  This property is required to enable the material to withstand the fatigue (weak) conditions (longer).

5          Simple design

Ø  In manufacture and installation are simplified to safe in costs.

Ø  Cast and welded joint

6          Access

Ø  Ease of access to the engine transmission system for inspection and maintenance on running gears purposes.

Ø  Inspection for crack at bearing pocket and welded joint also for tie bolts and holding down bolts etc.

7          Dimension

Ø  Ideally these should be as small as possible to keep engine containment to a minimum in order to give more engine room space.

Ø  However must can be fix with frame.

Ø  Ideally these should be as small as possible to keep engine containment to a minimum in order to give more engine room space.

Ø  However can be fixing with frame.

8          Seal

Ø  Must seal off effectively the oil and vapors by providing oil tight chamber and housing for thrust block

Ø  Any vapors or oil flowing out from it mean crack have been occur.

Faults in bed plates

The bedplate should be inspected regularly for evidence of cracking. This can occur in the welds joining the transverse girders to the longitudinal, and under the bearing pockets, where the cracks can be radial or follow the line of the pocket. As well as faulty manufacturing techniques, causes can be uneven loading/ overloading of engine units, loose tie bolts and loose holding down bolts.

Type of Faults

Ø  Cracks

Ø  Oil leaks

Ø  Loose chokes

Ø  Loose frames

Crack areas

 

Ø  Fabricated transverse girders often show cracks in welds around the bearing pockets.

Ø  Radically around tie bolt and frame bolt holes.

Ø  At the junction welds between fabricated cross girder and side girder.

Ø  Around lighting holes.

Ø  At the base of main bearing keeps.

Reason for failures or cracks

Ø  Excessive vibration

Ø  Slack tie bolts

Ø  Overloading due to excessive bearing wear

Ø  Poor welding or stress relieving

Ø  Stress riser (concentration) on weld

Repairs

Ø  Mild steel or cast steel materials – crack should be chipped out and welded. Care should be taken not to distort the welded part – repair to be done by ductile electrodes which will give deposit close to parent metal

Ø  Cast iron – Can be arrested by drilling (if small) and metallocked (if serious) and support by doubler-plate bolted on it

Bedplates inspection

Ø  Cracks

Ø  Corrosion – moisture or acidic compounds

Ø  Cleanliness – sludge and carbon

Ø  Loosen connections

Ø  Oil leaks

Ø  Faulty welding

Ø  Faulty casting

Ø  Cracks

Ø  Corrosion – moisture or acidic compounds

Ø  Cleanliness – sludge and carbon

Ø  Loosen connections

Ø  Oil leaks

Ø  Faulty welding

Ø  Faulty casting