effect of heat on alignment.doc

LESSON

2 LECTURE

HEAT AND ITS EFFECT ON ALIGNMENT

SUB-OBJECTIVE

At the end of this Lesson, the Trainee will be able to describe the effect of heat on alignment without error.

1.0 INTRODUCTION

The expertise required for aligning machinery is made even more complicated by the effects of heat. This is called Heat Growth, and needs to be taken into consideration during alignment operations.

2.0 PREPARATIONS

Before alignment is carried out it is important that all suction and discharge pipework is connected and other components installed, because if these are added after the alignment has been carried out it could very easily throw out the alignment again. The manufacturers specifications must be checked to see if any vertical difference between the coupling halves are specified because some equipment heats up as it runs. This is particularly important where a steam or gas turbine is to be the prime mover for the pump/compressor etc. This heating can raise the shaft during running, which is precisely when we require the alignment to be perfect!

Fig. 15-2-1.

This heat growth has to be taken into account when aligning the machine from cold. The data for this offset alignment is calculated by the manufacturer and is included in the handbook.

MACHINE ALIGNMENT MECHANICAL MAINTENANCELESSON 1 PAGE1 ADVANCED COURSE

TARIQ HUSSAIN SOOMRO (TRAINEE ENGINEER)

3.0 RUN-OUT

Another preparation that has to be checked before alignment can be carried out is called "Run-out".

This operation checks the concentricity of the shafts/couplings. This concentricity is checked using a dial indicator. The dial indicator is mounted on the base plate of the machine and set so that the needle is reading zero when touching the shaft/coupling. The shaft is rotated and the indicator observed to see if the permitted amount of deflection is not exceeded.

Any deflection means that the shaft is distorted or bent. This operations is similar to the exercise you did in Module 20 during the exercise of centering your work piece in the lathe chuck using the D.T.I. Time can be saved by placing the shaft on a precision surface during maintenance work and checking it out whilst it is still in the workshop using the D.T.I. or even Feeler Gauges whilst rotating the shaft.

Sketch what you understand by Runout.

4.0 CORRECTING VERTICAL MISALIGNMENT

All vertical misalignment, whether angular or parallel, is corrected by using shims. Shims are usually pieces of stainless steel or brass that are placed in pairs under the feet of equipment to correct vertical alignment. If, the gap at 12 O'clock is greater than that at 6 O'clock, then shims must replaced under the outboard feet of the motor to correct the vertical angular misalignment

Fig. 15-2-2If the problem was vertical parallel misalignment, then equally sized shims would be placed under all four feet of the motor.

MACHINE ALIGNMENT MECHANICAL MAINTENANCELESSON 1 PAGE2 ADVANCED COURSE TARIQ HUSSAIN SOOMRO (TRAINEE ENGINEER)

A DVANCED COURS

Fig. 15-2-3.

NOTE:

Since pumps and compressors are connected to pipe work, it is usually easier to move the motor on its bedplate because the motor is only constrained by flexible electric wiring. Moving the pump would mean?

There are some guidelines to follow when using shims:

Shims must always be a big as the foot of the equipment they support. It is better to use one thick shim rather than several thinner shims Shims should be clean and free of dirt, grease and paint. The bedplates and feet of the equipment must be clean and flat. Shims must always be used in pairs. Otherwise equipment could rock/tilt. Shims must not be liable to rust or corrode as this would upset the alignment.

5.0 CORRECTING HORIZONTAL MISALIGNMENT

Just as both types of vertical misalignment are corrected using shims, Horizontal Misalignment (both parallel and angular) can be corrected by moving the motor horizontally on its bedplate. In the case of horizontal parallel misalignment, the motor is unbolted from its bedplate, and the whole motor moved sideways.

Fig. 15-2-4.

If the problem is horizontal angular misalignment, the motor is unbolted and turned horizontally until the centre lines of both shafts meet to form a continuous straight line.



Fig. 15-2-5.

NOTE:

To enable the motor to be easily moved in the horizontal plane, the feet of the motor often have elongated or enlarged bolt holes.

6.0 DOWEL PINS

To ensure that once the motor has been aligned it will not move position due to vibration etc., and to enable easier alignment after motor removal, dowel pins are often fitted.

These may be of the tapered or parallel type. The fitting of these pins will be covered in workshop practical exercises.

Once alignment has been completed, it is now modern practice to cement around the machinery baseplates, particularly on heavier equipment.

7.0 ALIGNING BELT DRIVEN MACHINERY

Belt driven machinery is much easier to align because of its geometry, and because of a greater tolerance of its precision requirements.


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Fig. 15-2-6. Arrangement of motor slide rails.

8.0 TENSION SETTING

However, the setting of the tension of the belt is an operation that needs to be done accurately, and to the manufacturers specifications if long life of the bearings and belt are to be expected. This operation needs to be done when adjusting the belts on the air conditioning compressor, alternator and power steering pump of your car!



Fig. 15-2-7.

Examples of Uses of D.T.I.'s in Alignment Operations.

There are many methods of taking alignment readings, and calculating rectification corrections having obtained the readings by D.T.I., but they fall outside of the scope of this Module.

Indeed, Specialist Advanced Courses are required to learn some of the different methods than can be used.

The preparations for setting up the special brackets that are sometimes needed can be very time consuming.

Some of the brackets themselves introduce more complications to the alignment operations because they can introduce "Sag" into the calculations.

However, following are some figures that can give you an idea as to what can be involved when carrying out an alignment operation.

Your Instructor will give you a brief explanation as to the following methods of alignment using a D.T.I.Fig. 15-2-8. shows a D.T.I. attached to the shaft with a magnetic clamp.


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Fig. 15-2-8.

Fig. 15-2-9 shows the different position the D.T.I. takes as it moves up and down, and rotates around a shaft. You can imagine the deflection of the needle on the dial face as it is moved about.

Fig. 15-2-9.

Fig. 15-2-10 shows a method of alignment called the Face and Rim Method.



Fig. 15-2-10.

The method requires special brackets and the use of two dial indicators. The distances between the motor pedestal feet and the coupling half needs to be known as well as other measurements. Using special equations the required adjustments for perfect alignment can be calculated.

Fig. 15-2-11.Another method known as Reverse Dial Indicator can be used as shown in Fig. 15-2-12 below.


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Fig. 15-2-12.

This method also requires the use of special brackets and two DTI's. The alignment adjustment required can either be calculated using a formula, or it can even be done by using a geometric scale drawing on good quality graph paper.

Some alignment operations can taken days and even weeks to get perfect, but it is worth it in the end because the functional life of the machinery can meet its design specifications if done correctly.

9.0 END FLOAT

There is one other aspect of alignment that has not been mentioned yet, and that is End Float.

It is all very well aligning two shafts perfectly so that their centre lines are continuous, but it must not be forgotten that Axial Alignment is also very important.

In many cases it is vital that there is some room for axial expansion of the shafts.

This Axial Clearance is also very important for the Thrust Bearings of the machinery.



Fig. 15-2-13.

The Manufacturers specifications must be consulted so that the recommended End Float Clearance can be set correctly.


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QUESTIONS:

1. Name three precision alignment instruments

2. How can heat affect alignment?

3. How do you compensate for heat when aligning from cold?

4. What does a "Runout reading tell you?

5. Name four guide lines to be used when using Shims.

6. How is vertical misalignment corrected?

7. How is horizontal misalignment corrected?

8. Why are Dowel pins used?

9. On a belt driven machine, what would be the effect if the belt tension was:

a. Too lowb. Too high

10. Name two methods of taking alignment readings using D.T.I.'s

11. What is "Sag"?

12. What is End Float?



effect of heat on alignment.doc

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