DIFFERENT TYPES OF FITS

Fits:

          The relationship between two parts, which are to be assembled, with respect to difference of their sizes, before assembly is called 'Fit'.

          It is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in  mating parts.

There are four types of fits between parts, they are

          1. Clearance Fit

          2. Interferance Fit

          3. Transition Fit

          4. Line Fit

1. Clearance Fit:

          When an internal member fits in an external member (as a shaft in hole), in clearance fit there is always an air space or clearance between the shaft and hole. Such type of fit give loose joint. A clearance fit always have a positive allowance.

Clearance fit also sub-classified as

a) Slide fit

b) Running fit

c) Slace running fits

d) Loose running fits

2. Interference Fit:

          When the internal member is larger than the external member then there is always an actual interferance between those materials. This negetive difference between diameters of shaft and hole  is called interference. An interferance fit always have a negetive allowance.

          From the below example, the smallest size of the shaft is 2.2313" and the largest hole is 2.2305" so that least interference among the materials is 0.0008". Under maximum material condition the interference is 0.0018".  

Interferance fit can be sub-classified as

a) Shrink fit (or) Heavy force fit

b) Medium force fit

c) Tight fit (or) Press fit.

3. Transition Fit:

          It may result in either Clearence fit or Interference fit condition depending on the actual value of the individual tolerances of mating parts. This type of fit is used for applications where accurate location is important but either a small amount of clearence or interference is acceptable. 

Transition fit can be sub-classified as 

a) Push fit

b) Force fit

c) Wringing fit

4. Line Fit:

          The limits of size are so specified that a clearance or surface contact may result when mating parts are assembled.

ex: Shrink fit   

DEFINITIONS OF LIMITS, FITS & TOLERANCES

Limits: 

          A useful definition is provided by McGraw Hill: "Two extreme permissible sizes of a part between which the actual size is contained, are called Limits".

Fits:

          The relationship between two parts, which are to be assembled, with respect to difference of their sizes, before assembly is called 'Fit'.

          It is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in  mating parts.

Tolerance:

          The total permissible variation of a size is called Tolerance. It is the difference between maximum and minimum limit of size.

High consistancy (Hc) cleaners | | working principle of high consistency cleaner

High consistancy (Hc) cleaner is made ready according to foreign products and relevent technical information. It is used for removing high density particles like sand, metalic pieces (staples, paper clips) etc., from pulp in pulping and paper industry. These particles may cause wear or damage to subsequent machinery and therefore they must be removed from pulp. It is one of the main equipment in wastepaper recycling and pulping industry.

It is placed vertically, it can takes up less area. The cleaner can be directly mounted on floor and fixed on rigid support as well. It is very easy to operate and maintain the cleaner.

Basic Parts:

The cleaner mainly consists of; inflow pulp chamber, cone, support, dregs tank.

The pulp inlet line is present at the top of the cleaner in tangancial direction, accepted pulp outlet line is present at the centre of the top of the inflow pulp chamber with an inserted wear resisting sleeve. The cone is devided into two parts as upper section and lower section. Upper part is pulp section and lower part is dregs section. The balancing water inlet line is present at the top of dregs section and washing water inlet line is present in dregs section. Sight glass is also present in the dregs tank for observation.

Working Mechanism:

Pulp goes into the cleaner along a tangent line at a pressure 0.2 to 0.6 MPa. When the pulp moves in spirals along the internal walls, accelerative centrifugalisation occurs. Because of this centrifugalisation, heavy particles present in the pulp are thrown towards the internal walls and move downwards in spirals. At the center of the area near the smaller diameter end of the top cone, the accepted pulp in spiral motion forms eddy under the interaction of speed and the motion area. Simulatniously the accepted pulp forms an up screw-column motion and flow outwards through the outlet present at the top of the cleaner. Heavy particles go downwards to  the bottom of the cone and settle in the dregs tank. This is connected to a high pressure water line, for washing the heavy particles and recovering the fibre. Under low consistancy and high-pressure difference, effeciency of cleaner to remove heavy particles is high.

Main specifications and technical parameters:

We are giving what are the specifications and technical parameters we have to consider before implementing the cleaner.

Model
Flow rate : L/min
Consistancy % of inflow pulp
Pulp inflow pressure: MPa
Difference of inflow and outflow pulp pressure: MPa
Pressure of equilibrating water: MPa
Flow of equilibrating water : L/Min
Pressure of compressed air: Mpa
Rejects discharge method	Automatically/Manually
Weight of equipment

Operation:

1. Open the valve of the accepted pulp outlet, close the upper and under gate valves of the dregs chamber.

2. Open the balancing and washing water valves and fill water into the drehs tank.

3. Open the upper valve of dregs tank, then the washing water raises continously.

4. Start to supply pulp, and ensure the pressure difference between the inlet and outlet flow pulp.

5. Balancing water pressure must be above the inlet pulp flow pressure, if the pressure is low adjustment should be done.

6. After settledown of dregs in dregs chamber we have to discharge them periodically. To do this we have to close the washing water valve and upper valve of dregs tank, and then open the the lower valve of dregs tank to discharge the dregs.

CENTRIFUGAL MONOBLOCK PUMPS

          In Monoblock Pumps, Centrifugal monoblock pump is one of the common types. In a lot of places centrifugal pumps offers more benefits. Here are the benefits offered by these pumps:

1) Due to its compact size, it consumes less space.

2) There is no air lock problem in these types of pumps, because these have uni casing.

3) There is no centrifugal switch.

4) The pump has a balanced structure so there is no vibration and noise. Rigged construction that ensures longevity.

5) There is less maintenance & operation cost.

6) Bearings of the pump are water lubricated.

7) These pumps are self-supportive. This means that there is no requirement of installing any base plate or guards.

8) These pumps have the best hydraulic and suction lifting efficiency.

9) They are self priming. They are also put into use to pump water from well (Both shallow and deep wells) with built in Non return valves.

Applications:

          These pumps can be used at a variety of places to meet different water requirements. Below we mentioned some applications of these type of pumps:

a) Industrial and Public water supply.

b) Booster application

c) Sprinkler/Drip and Lift Irrigation

d) Dewatering of mines.

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., unitless) 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.  

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 aliging the shafts to precesion alignment is necessary. beacuse, by this we will increase the machine life,  decrease vibrations, reduce energy consumption and also the couplings life.