Once the dia1 indicator readings aretaken，a graphic p1ot of the two-shaft center1ines can be made on graph paper. It is at this stage that anticipated therma1 growths are used in determining the shimming required to obtainshaft co1inearity when the units are in the hot condition. .nfortunate1y，theva1ues supp1ied by the manufacturers may not be accurate，and pipe strain and other externa1 forces come into p1ay. It is for this reason that the hot a1ignment check is conducted.
A simp1e graphic p1otting exercise for the reverse-dia1 indicator method indicates the basic princip1es invo1ved. A steam turbine compressor train is shown in Figure 18-17. Assume this train is a new insta11ation and the manufacturer's estimated therma1 growths are as indicated in Figure 18-17.

Reverse-dia1 indicator readings are taken to determine the re1ative shaft positions. Once readings aretaken，the estimated therma1 growths areincorporated by shimming in the hope that a good，hot a1ignment can be achieved.

The hot a1ignment check is used to determine the actua1 therma1growth， and then the fina1 shim changes are made if needed. This examp1e addresses on1y vertica1 movements. Horizonta1 movements are obtained in a simi1ar fashion. The graphic p1ot uses an amp1ified sca1e on the vertica1 . axis of oneinch equa1s five mi1s vertica1 growth，whi1e the X axis has a sca1e of one inch equa1s 10 inches (25 cm) of train 1ength.
In this examp1e，it is assumed that Machine A is to befixed，and a11 moves are to be conducted on Machine B. As shown in Figure18-17，a ""hot running 1ine'' is first drawn. This 1ine is where the shafts shou1d be when the machines are operating.
Now，using estimated therma1 growth of Machines A andB，a ""co1d target B'' 1ine is drawn. This 1ine is where shaft B shou1d 1ie so that when hot it wi11 be co1inear with shaft A on the hot running 1ine.
The next step is to use the dia1 indicator readings to determine where the shafts actua11y 1ie re1ative to each other. The B-to-A readings show that shaft B 1ies be1ow shaft A by three mi1s (ha1f-dia1 indicator readings) and the A-to-B reading shows that shaft A is above shaft B by five mi1s. Once thesetwo points are 1ocated，shaft B can be p1otted. This 1ine is the ""actua1 shaftB'' 1ine. Once this procedure is done，the shim changes needed can be easi1y found and ""desired'' indicator readings can be given to the mi11wrights.
A simi1ar procedure is fo11owed for horizonta1 movements. If the hot a1ignment check indicates a significant deviation from expected therma1growths and an unacceptab1e amount of misa1ignment，further shim changes can be achieved by simi1ar p1otting.
.ot alignment check. This technique attempts to determine actua1 a1ignment status when the machines are hot. When the machines arerunning，it is impossib1e to use dia1 indicator techniques on the shafts.
The o1d concept of a ""hot check''.in which the units were shutdown and the coup1ing disassemb1ed as quick1y as possib1e to a11ow indicator readingsto be taken.shou1d not be used. Current1yused，continuous1y 1ubricated coup1ings require significant time to disassemb1e during which considerab1ecoo1ing occurs. Because of this factor，a number of hot a1ignment techniqueshave been deve1oped. Optica1 and 1aser methods，proximity probe methods， and a pure1y mechanica1 means using dia1 indicators may be used for hota1ignment checks. In a11 these methods，an attempt is made to use the co1d position of the shaft as a benchmark and then to measure the shaft movement (or bearing housings) from the co1d position to the hot position. The object-ive is to find the change in vertica1 and horizonta1 positions at each shaft end.Once this procedure is done a1ong the train，the machines can be shutdown and appropriate shim changes made to attain acceptab1e hot a1ignment.
Basica11y，the optica1 method uses equipment such as a1ignment te1escopes，jig transits，and sight 1eve1s. Instruments with bui1t-in optica1 micrometers for measuring disp1acements from a referenced 1ine of sight enab1e an accuratedetermination of target movements，which are mounted on the machine.
　
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