CAE DS – Injection Casting Design Press Fits Technical University of Gabrovo – Yordanka Atanasova The surfaces of the parts making up a joint are called enveloping (hole) and envel‐ Definitions and oped (shaft), and their diameters, enveloping (hole diameter) and enveloped (shaft dependences diameter), respectively. The difference between the enveloping and enveloped dimension determines the nature of the joint and is called a fit. The fit shows the greater or smaller freedom of relative motion of the parts and the strength of their immovable joint. When the hole dimension is greater than the shaft dimension, their positive differ‐ ence is called clearance (J). When the hole dimension is smaller than the shaft dimension, the absolute value of their negative difference is called interference (S). The type of the fit depends on the size and the mutual location of the margin toler‐ ances of the hole‐shaft joint. Depending on this, fits are divided into three groups (Fig.1): a) Clearance fits – they guarantee the joint clearance; b) Press fits / interference fits – they guarantee the joint interference; c) Transition fits – depending on the size and the location of the margin toler‐ ance, there can be either interference, or a clearance in the joint. It is important to form the different fits in such a way that one of the pair of parts, called basic part, has a margin tolerance which is constant in value and location for all fits of a certain grade and a certain range of dimensions. When the basic hole is permanent, then the system of fits with different clearances and interferences is produced by fitting different shafts with this hole. In this case the system is called basic hole (SAN). Production of When the basic shaft is permanent, then the system of fits with different clearances press fits of and interferences is produced by fitting different holes with this shaft. In this case plastic joints the system is called basic shaft (SArN). There are different grades in the standard system of tolerances and fits, depending on the dimension of the tolerance. In each grade the tolerance changes approxi‐
mately in proportion to D⅓. To avoid the need for having an individual margin tolerance for each dimension, the entire scope of dimensions is broken into ranges. The same tolerance is assumed for all diameters in each range and it is calculated using the mean value of the diameter. The standards cover the three ranges of dimensions: from 0 to 500 mm, from 500 to 3150 mm and from 3150 to 10000 mm. The tolerances and fits discussed in the standards refer to parts measured at a temperature of 200С. Products made of polymer materials are joined together or with other materials by means of press fits. They are produced by pressing in or as the plastic shrinks while cooling down. Press fits ‐ 1 CAE DS – Injection Casting Design When a joint is produced by pressing in (as is the case with metal products), there is a risk of damaging the surface of the products being joined. Therefore, press fits in plastic products are more often produced by shrinking the product with a hole, for example, a bush. Basic hole system, Basic shaft system Press fits Transition fits Running
fits
Press fits Margin tolerance of the hole
Transition fits Running
fits
Margin tolerance of the shaft;
Fig. 1. Groups of fits Press fits of plastic products do not require big efforts, due to the smaller resistance as a result of the low coefficient of dry friction for polymer materials. To produce a press fit, the bushes are heated to a temperature depending on the material type / Table 1/ The size of the bush and the interference value can be calculated using the data in Table 1. Table 1 Material Linear expan‐
sion coefficient, α х 10 ‐6 оС‐1 Heating tempera‐
ture, оС Linear shrink‐
age, % ABS 80 90 ‐ 120 0,6 PMMA 70 70 ‐ 100 0,2 PEHD 180 90 ‐ 130 2,0 PELD 220 80 ‐ 110 1,8 PP 150 70 ‐ 120 1,7 Bushes made of polymer materials must have interference greater than the calcu‐
lated value, since the actual interference, due to the considerable deformability of these materials, especially when heated, is lower. Dimensioning The specific features of plastics as structural materials are manifested especially when determining the fits of the products and the tolerances of their dimensions. Press fits ‐ 2 CAE DS – Injection Casting Design Table 2. Margin tolerances for plastic products Press fits ‐ 3 CAE DS – Injection Casting Design Table 3. Recommended fits for plastic products Press fits ‐ 4 CAE DS – Injection Casting Design The conditions under which these tolerances are valid are temperature +20°С; relative humidity of air 65%; door‐to‐door time (before taking measurement) — 12h grades up to JT12, 6h for JT13 and JT14, and 3h for JТ15 and higher. Some of the margin tolerances for plastic products regulated by the standard are given in Table.2. Additional margin tolerances ay, az and ze for shafts and АУ, AZ and ZE for holes are envisaged for specific occasions, for example great linear‐
shrinkage coefficient. Recommended fits for plastics to plastics are given in Table.3. The following margin tolerances are recommended for fitting plastic to metal products: − for shafts — h7, h8, h9, hl0, h11, hl2; − for holes — H7, Η8, Η9, Н10, Н11, Н12 Press fits ‐ 5