Engine frame and Tie rod
Engine frame
A Frames are used to support the cylinder block in most two stroke main propulsion engines. They are called A Frames, because they resemble the letter 'A' in shape. They are fitted at each transverse girder, starting from the ends.These are usually fabricated from steel plates.
In superlong stroke engines, high structural rigidity is of major importance. Extensive stress and deformation calculations are carried out, using full three-dimensional finite-element Computer models for different column designs, to verify optimum frame configuration. The Bedplate and the Columns should be designed for minimum machining requirements. The cylinder jacket can be built for a single unit, or may include more cylinders, depending upon the manufacturer's casting capacities.
Tie rods
The Cylinder entablature, 'A' Frames and the Bedplate are bound firmly together, by long steel Tie-rods, passing through hollow columns. The Tie rods are of ordinary Mild steel, and screwed at each end to take the nuts. The lower nut is squared and fits into an opening of similar shape cast in the Bedplate, so as to prevent the nut from turning, when the bolt is screwed into it, and the upper nut is tightened.
For a 9 m height engine, the Cast iron parts of the engine would be compressed from 0.5-0.75 mm by tightening the Tie bolts.
The Tie rods are pre-stressed at assembly, so that the Engine structure is under compression at all times. Two Tie-rods are fitted to each transverse member, and pass through tubes.
In large super long stroke low speed propulsion engines, the Tie rods may be in two parts, to facilitate ease of removal. To prevent any lateral movement which could cause vibration problems, 'pinch' bolts are fitted.
Tie rod tensioning
The Tie rods need to be checked for proper tension at intervals laid down in the maintenance schedule, and also after any scavenge fire. This is done hydraulically for modern engines.
Large tie-bolts are tightened with a hydraulic jack which loads the tie-bolt in tension. The tie-bolt nut is usually drilled to take a toggle bar or, slotted to take a hook spanner, and when the correct pull is on the tie-bolt, the tie-nut is pulled up hand-tight; the pressure in the jack is then released leaving the tie-bolt tight. The load placed on the tie-bolt by the hydraulic jack is controlled by the hand-pump pressure, which is indicated on the pump pressure gauge.
This is done by. the following method :
1. Connect two pre-tensioning jacks to two adjacent tie bolts.
2. Obtain the required hydraulic pressure and maintain this.
3. Check the clearance between the nut and the intermediate ring and adjust if required.
4. Release the pressure and remove the jacks.
By pre-tensioning the Tie rods, a pre-determined tensile load is induced in the Tie rod, and a pre-determined compressive load is induced in the Entablature, 'A' Frame and Bedplate. During the firing stroke, the tensile load on the Tie rod increases.
The compressive stresses on the Entablature, 'A' Frame and Bedplate are reduced, thereby keeping the components at very low fatigue levels. In other words, if the Tie rods were not pre-tensioned properly, the fluctuation of stresses in the components between the firing and the non-firing periods could be higher; leading to fatigue failure of components.
To minimise bending, Tie rods are placed as close to the Crankshaft axis as possible. This reduces the Bending stress on Girders and prevents unbalanced loads from being transmitted to the welds.
Loose and broken Tie rod.
How to find loose tie-rods, while the engine is running.
If tie-rods are not taking their proper load due to slackened nuts, the cylinder jackets adjacent to the slack tie-bolts can be seen lifting, when the piston is nearing the end of compression, or on firing. If no movement is visible, the thumb can be pressed on the cylinder jacket, with the thumb nail in contact with the tie-bolt nut.
Small movements, too small to be visible, can be felt with the thumb nail. If washers are fitted between tie-bolt nuts and cylinder jackets, a washer may sometimes be twisted at the end of the firing strokes in each cylinder adjacent to the slack tie-bolt.
Dial indicator gauges can also be used to detect relative movement between the tie-bolts and cylinder jackets. Tie-bolts should be kept under observation at all times, particularly when running at reduced power in bad weather and following power increases during moderating weather.
Fretting of landings of the cylinder beam on the A-frames and of top tie bolt nut landings are signs of loose tie rods. Fretting of a surface is often indicated by a rust-red powder being present at the outside of the faces that are fretting. If the location is dry and free of oil, the red powder locks as if it has been dusted on the surfaces, being thickest at the joint.
Consequences of running an engine with slack Tie-rods If an engine is run with slack tie-rods, the cylinder beam flexes and lifts at the location of the slack tie-bolt. In time the landing faces of the tie-bolt upper and lower nuts, and the landing faces of the cylinder beam on the A-frame, fret and the machined faces are eventually destroyed.
The fitted bracing bolts between the cylinder jackets will also slacken and the fit of the bolts will be lost. If fretting has occurred in an uneven pattern, where the cylinder beam lands, and the tie-bolts are tightened, the alignment of the cylinders to the line of the piston stroke is destroyed. After fretting has occurred, nut landing faces may be out of square, and if tie-bolts are tightened on faces which are out of square, a bending moment will be induced in the tie- bolt. This, in turn, causes an uneven stress pattern in the tie-bolt, which could lead to early fatigue failure.
If tie-rods are not taking their proper load due to slackened nuts, the cylinder jackets adjacent to the slack tie-bolts can be seen lifting, when the piston is nearing the end of compression, or on firing. If no movement is visible, the thumb can be pressed on the cylinder jacket, with the thumb nail in contact with the tie-bolt nut.
Small movements, too small to be visible, can be felt with the thumb nail. If washers are fitted between tie-bolt nuts and cylinder jackets, a washer may sometimes be twisted at the end of the firing strokes in each cylinder adjacent to the slack tie-bolt.
Dial indicator gauges can also be used to detect relative movement between the tie-bolts and cylinder jackets. Tie-bolts should be kept under observation at all times, particularly when running at reduced power in bad weather and following power increases during moderating weather.
Fretting of landings of the cylinder beam on the A-frames and of top tie bolt nut landings are signs of loose tie rods. Fretting of a surface is often indicated by a rust-red powder being present at the outside of the faces that are fretting. If the location is dry and free of oil, the red powder locks as if it has been dusted on the surfaces, being thickest at the joint.
Consequences of running an engine with slack Tie-rods If an engine is run with slack tie-rods, the cylinder beam flexes and lifts at the location of the slack tie-bolt. In time the landing faces of the tie-bolt upper and lower nuts, and the landing faces of the cylinder beam on the A-frame, fret and the machined faces are eventually destroyed.
The fitted bracing bolts between the cylinder jackets will also slacken and the fit of the bolts will be lost. If fretting has occurred in an uneven pattern, where the cylinder beam lands, and the tie-bolts are tightened, the alignment of the cylinders to the line of the piston stroke is destroyed. After fretting has occurred, nut landing faces may be out of square, and if tie-bolts are tightened on faces which are out of square, a bending moment will be induced in the tie- bolt. This, in turn, causes an uneven stress pattern in the tie-bolt, which could lead to early fatigue failure.
Breakage of tie-rods
If, during an inspection, a Tie-rod is found to be broken, it must be replaced, as soon as practicable. If the breakage leaves the lower portion short- remove it through the Crankcase and withdraw the upper part from the top. If the breakage leaves a long lower portion, the Tie rod should be cut and removed in sections through the Crankcase.
If, during an inspection, a Tie-rod is found to be broken, it must be replaced, as soon as practicable. If the breakage leaves the lower portion short- remove it through the Crankcase and withdraw the upper part from the top. If the breakage leaves a long lower portion, the Tie rod should be cut and removed in sections through the Crankcase.
These bolts are under repeated applied stress. The various stresses undergone by the bottom end bolts are shown below.
Reversed Stress: The stress range is symmetrical about the zero stress line.
Repeated Stress: The Bottom end bolt is stressed and then completely unloaded.
Fluctuating Stress: The Bottom end bolt is stressed either compressive or tensile, but the stress range does not pass through zero.
Alternating Stress : The Bottom end bolt undergoes a stress range, which passes through the zero stress line. Hence it changes from Tensile to Compressive. But it is asymmetrical about the zero stress line. These ranges of stresses passing through the zero line can have the effect of lowering the life span of the component, for the same stress range.
It is prone to fatigue failure : One portion of the fracture will be discoloured, and relatively smooth, whereas the other portion will be clean or crystalline. The fatigue limit is increased by using alloy steels, and is reduced by changing the section of the bolt, fillet and so on.
Reversed Stress: The stress range is symmetrical about the zero stress line.
Repeated Stress: The Bottom end bolt is stressed and then completely unloaded.
Fluctuating Stress: The Bottom end bolt is stressed either compressive or tensile, but the stress range does not pass through zero.
Alternating Stress : The Bottom end bolt undergoes a stress range, which passes through the zero stress line. Hence it changes from Tensile to Compressive. But it is asymmetrical about the zero stress line. These ranges of stresses passing through the zero line can have the effect of lowering the life span of the component, for the same stress range.
It is prone to fatigue failure : One portion of the fracture will be discoloured, and relatively smooth, whereas the other portion will be clean or crystalline. The fatigue limit is increased by using alloy steels, and is reduced by changing the section of the bolt, fillet and so on.
Life of Bolts
If the Bottom end bolt has completed approximately 18000 to 20000 running hours (4 stoke), it must be renewed, whatever its condition. They should also be renewed if a piston seizure takes place or a failure of the over- speed trip.
For large highly stressed Bottom end bolts, the traditional means of tightening by hammer has been replaced by hydraulic tightening, which ensures more uniform tightening. There is no shock loading and bending moments (twist).
If the Bottom end bolt has completed approximately 18000 to 20000 running hours (4 stoke), it must be renewed, whatever its condition. They should also be renewed if a piston seizure takes place or a failure of the over- speed trip.
For large highly stressed Bottom end bolts, the traditional means of tightening by hammer has been replaced by hydraulic tightening, which ensures more uniform tightening. There is no shock loading and bending moments (twist).
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