2.    Type of Fits

a.General. Information concerning fits will be applied to plain cylindrical parts such as sleeves, bearings, pump wearing rings, and other non­threaded round parts that fit together. Fit is defined as the amount of tightness or looseness between two mating parts when certain allowances are designed in them. An allowance is the total difference between the size of a shaft and the hole in the part that fits over it. This allowance and the resulting fit can be a clearance (loose) fit, an interference (tight) fit, or a transitional (somewhere between loose and tight) fit. These three general types of fits are further identified by classes of fits, with each class having a different allowance, based on the intended use or function of the parts involved. A brief description of each type of fit will be given in the following paragraphs. Any good handbook for machinists has complete charts with detailed information on each individual class of fit. Most major types of equipment repaired in machine shops will have the dimensional sizes and allowances noted, on blueprints, or in the appropriate manufacturer's technical manual.

b.Clearance Fits. Clearance fits or running and sliding fits provide a varying degree of clearance (looseness) depending on which one of the nine classes is selected for use.The classes of fit range from class 1 (close sliding fit) to class 9 (loose running fit) . A class 1 fit permits a clearance allowance of from +0.0004 to +0.0012 inch on the mating parts with a 2.500 inch basic diameter. A class 9 fit permits a clearance allowance of from +0.009 to +0.0205 inch on the same parts. Even for a small basic diameter (2.500 inch) clearance allowance from a class 1 minimum to a class 9 maximum differs by +0.0201 inch. As the basic diameter increases, the allowance increases. Although the class of fit may not be referenced on a blueprint, the dimensions given for the mating parts are based on the service performed by the parts and the specific conditions under which they operate, as described in each of the class of fits. Some parts that fall within these classes of fits are a shaper ram (close sliding), a babbitt­lined bearing, and pump wearing rings (loose removal).

c.Transitional Fits. Transitional fits are subdivided into three types known as locational clearance, locational transitional, and locational interference fits. Each of these three subdivisions contain different classes of fits. These classes provide either a clearance or an interference allowance, depending on the intended use and class selected. All of the classes of fits in the transitional category are primarily intended for the assembly and disassembly of stationary parts. Stationary means that the part will not rotate against another, although they may rotate together as part of a larger assembly. The allowances used as examples in the following descriptions of the various fits represent the sum of the tolerances of the external and internal parts. To achieve maximum standardization and to permit common size reamers and other fixed sized boring tools to be used as much as possible, use unilateral tolerance method.

(1)   Locational Clearance Fits.These are broken down into 11 classes of fits. The same basic diameter with a class 1 fit ranges from a zero allowance to a clearance allowance of 0.0012 inch. A class 11 fit ranges from a clearance allowance of +0.014 to +0.050 inch. The nearer a part is to a class 1 fit, the more accurately it can be located without the use of force.

(2)   Locational Transitional Fits. These type of fits have six different classes which provide either a small amount of clearance or an interference allowance, depending on the class of fit selected. The 2.500 inch basic diameter in a class 1 fit ranges from an interference allowance of ­0.0003 inch to a clearance allowance of +0.0015 inch. A class 6 fit ranges from an interference allowance of ­0.002 inch to a clearance allowance of +0.0004 inch. The interference allowance fits may require a very light pressure to assemble or disassemble the parts.

(3) Locational Interference Fits.These fits are divided into five different classes, providing an interference allowance of varying amounts. A class 1 fit for a 2.500 inch basic diameter ranges from an interference allowance of ­0.0001 to ­0.0013 inch. In comparison, a class 5 fit ranges from an interference allowance of from ­0.0004 to ­0.0023 inch. These classes of fits are used when parts must be located very accurately while maintaining alignment and rigidity. They are not suitable for applications where one part is subjected to a force that causes it to turn on the other part.

d.Interference Fits. There are five classes of fits within the interference type.They are all fits that require force to assemble or disassemble parts. These fits are often called force fits. In certain classes of fits, they are referred to as shrink fits. Using the same basic diameter as an example, the class 1 fit ranges from an interference allowance of ­0.0006 to ­0.0018 inch. In comparison, a class 5 fit ranges from an interference allowance of ­0.0032 to ­0.0062 inch. The class 5 fit is normally considered to be a shrink fit class because of the large interference allowance required.

(1)   A shrink fit requires that the part with the external diameter be chilled; or that the part with the internal diameter be heated. One can chill a part by placing it in a freezer, packing it in dry ice, spraying it with CO2 (do not use a CO2 bottle from a fire station) or by submerging it in liquid nitrogen. All of these methods except the freezer are potentially dangerous, especially the liquid nitrogen. They should not be used until all applicable safety precautions have been reviewed and implemented. When a part is chilled, it actually shrinks in size a certain amount depending on the type of material, design, chilling medium, and length of time of exposure to the chilling medium. A part can be heated by using an oxyacetylene torch, a heat­treating oven, electrical strip heaters, or by submerging it in a heated liquid. As with chilling, all applicable safety precautions must be observed. When a part is heated, it expands in size, allowing, easier assembly. All materials expand a different amount per degree of temperature increased. This is called the coefficient of expansion of a metal. It is important to determine the maximum temperature increase required to expand the part for the amount of shrinkage allowance, plus enough clearance to allow assembly. Overheating a part can cause permanent damage and produce so much expansion that assembly becomes difficult.

(2)   A general rule of thumb for determining the amount of interference allowance on parts requiring a force or shrink fit is to allow approximately 0.0015 inch per inch of diameter of the internally bored part. There are, however, many variables that will prohibit the use of this general rule.

(a) The amount of interference allowance recommended decreases as the diameter of the part increases.

(b)   The dimensional difference between the inside and the outside diameter (wall thickness) also has an effect on the interference allowance.

(c)   A part that has large inside and outside diameters and a relatively thin wall thickness will split if installed with an excessive interference allowance.

When there are no blueprints or other dimensional references available, all of these variables must be considered before a fit is selected.