Is it necessary to Align A Flexible Coupling?

          When I started my career as a maintenance engineer i got a doubt, is it necessary to align flexible coupling too?? we are using flexible coupling instead of fixed or rigid couplings because they don't have to be aligned??.

Later I read some articles and found some useful information. Today I wanna share this information with you people.

Courtecy : T B Woods Inc

For every coupling its manufacture give some common value of alignment specification:

According to T.B Woods Sureflex Coupling (with an 8je insert) 

--> Cost : about $XXX per sleeve or insert

--> Maximum  RPM : 4500

--> Parallel Misalignment : 0.020 in

--> Angular Misalignment : 0.094 in (about 0.012"/inch)

          Based on these manufactures recommendations the coupling can bear or tolerate some sort of misalignment. and also the insert (i.e rubber) is inexpensive then why we have to go precision alignment??

          Because we are aligning shafts -- we are not aligning couplings. The coupling sleeve or Insert that we use can tolerate that misalignment but the bearings on the shafts, oil seals, gears and also shafts can't tolerate.

Sometimes coupling will bend, but also it resists bending, for this it need some good amount of energy to oppose that force. That enegy is produced within itself by:

1) Increased heat due to friction

2) Increase in load on bearings, shafts, and seals.

          So finally i got to an conclusion, "is it necessary to align a flexible coupling?"- we should not worry about it but aligning the shafts to precision alignment is necessary. because, by this we will increase the machine life,  decrease vibrations, reduce energy consumption and also the couplings life. 

ALIGNMENT CONDITIONS

In general there are 4 alignment conditions:

           1) Perfect alignment

           2) Parallel or offset misalignment

           3) Angular or Face misalignment

           4) Skewed or combination misalignment (i.e., offset and parallel)

PERFECT ALIGNMENT :

          When the two shafts are perfectly aligned they will be colinear and operate as a solid shaft when coupled. Without performing alignment procedure we saw rarely perfectly aligned shafts. Regular checking is necessary to maintain the perfect alignment condition. Regular checking is the key condition in preventing maintenance.

Image Credits: www.myodesie.com

OFFSET or PARALLEL MISALIGNMENT :

          When there is a distance between shaft center lines, it is referred as Offset or parallel misalignment. Generally this alignment is measured in thousandths of an inch. In this condition the shafts are parallel to each other but not colinear. Offset may present in either the vertical or horizontal plane. Theoretically, offset is measured at coupling center line.

ANGULAR MISALIGNMENT :

          Angular misalignment also referred as face misalignment. Angular misalignment is the condition when the shafts aren't parallel (Angular offset) however they are in same plane with no offset. Angularity is the angle between the two shaft centerlines, which usually expressed as a "slope", or "rise over run" of so many thousandths of an inch per inch (i.e., unit less) rather than as an angle in degrees. It should be determined both in the vertical and horizontal planes.

COMBINATION OR SKEWED MISALIGNMENT :

          Skewed or Combination misalignment  occurs when both Offset and Angular misalignment occurs. In this type of misalignment shafts are not parallel and also they wont intersect at the coupling. This type of misalignment can occur in both the horizontal and vertical planes or either in the horizontal or vertical planes.  

SHAFT ALIGNMENT - ALIGNMENT PLANES

          Before we start learning alignment this is very important to know the types of misalignment planes and types of misalignment.

Generally there are two misalignment planes that we have to correct: 

          1) Vertical and 

          2) Horizontal.

In general there are two types of misalignment.

          1) Parallel (also known as Offset)

          2) Angular (also known as Gap).

Therefore, in the case of making a machine-train by using at least two machines, four types of misalignment can occur: 

          a) Vertical Offset

          b) Vertical Angularity

          c) Horizontal Offset, and

          d) Horizontal Angularity.

These can occur in any combination and in many cases, all four are present.

Vertical:

          Vertical misalignment means that the misalignment of the shafts in the vertical plane. In vertical misalignment both angular and offset misalignment can occur.

          Offset (Parallel) vertical misalignment happens when the motor shafts is moved vertically away from the pump shaft, but both shafts still operate in the same vertical plane and parallel.

          Gap (Angular) vertical misalignment occurs when the motor shaft is at an angle with the pump shaft, However, both shafts still operate within the same vertical plane.

          Vertical misalignment, which is corrected by the employment of Shims.

MIS-ALIGNMENT PLANES-- VIEW

Horizontal:

          Horizontal misalignment means that the misalignment of the shaft in the horizontal plane. Just like vertical misalignment, in horizontal misalignment also both angular and offset misalignment can occur.

          Offset (Parallel) horizontal misalignment happens when the motor shaft is moved horizontally away from the pump shaft, but both shafts still operate in the same horizontal plane and parallel.

          Gap (Anhular) horizontal misalignment occurs when the motor shaft is at an angle with the pump shaft. However both shafts operate within the same horizontal plane. 

          Horizontal misalignment, which is not corrected by employing Shims. This type of misalignment is corrected by physically moving the MTBM.

Alignment Tolerances:

ALIGNMENT TOLERANCES

SHAFT ALIGNMENT

Shaft Alignment - Fundamentals

          Shaft alignment is process of positioning (or) aligning of shaft center lines of the driven and driver components (i.e., gearboxes, pumps, rolls etc.,) within a tolerated margin. Alignment is accomplished by giving shimming or moving machine components or sometimes both. The aim of shaft alignment is to maintain a common axis of rotation at operating equilibrium for two coupled shafts.

          In case of high speed equipment if the shaft alignment is not perfect we cant get maximum reliability and life through that equipment. Alignment is important in the case of direct coupled shafts and also the shafts of machines that are separated by distance even though we use flexible coupling. Shaft alignment is very important why because due to misalignment a lot of stress on shafts, vibration and noise produces. This causes excessive wear, bearing problems like heating and bearing failure, and results in the need for frequent repairs. By proper alignment we can reduce noise, vibration and also power consumption and this helps to achieve the design life of bearing, seal, shaft and coupling. 

          Alignment procedure is done by assuming a machine component is stationary, level and supported properly its base plate and foundation. Both angular and offset alignment must be performed in the vertical and horizontal planes, which is obtained by moving horizontally or by raising or lowering to align with the rotational center line of the stationary shaft. The movable components are designed as Machines To Be Moved (MTBM) or Machines To Be Shimmed (MTBS). MTBM generally refers to correction in horizontal planes and MTBS refers to corrections in vertical planes.