Aim of the fitting system
The aim of the fitting system is to ensure that at least one of the components is manufactured as uniformly as possible. If particularly high demands are placed on the tolerances, the other component can be used. For the sake of simplicity, the bore is the preferred side for the fit. The shaft is usually detachable and therefore easier to rework. Accordingly, the “bore” unit system applies to the bore and vice versa.
Standardized production with the standard bore and standard shaft fitting system
DIN 7155 is the basic standard for ISO fits. It also includes the system for the shaft. This standardization allows products to be manufactured interchangeably, even if they come from different manufacturers. This interchangeability and requirement for identical production makes it possible for mechanical engineering companies to network with each other. It also reduces costs and enables repeatable production with consistent, verifiable quality. Larger tolerances are ruled out by the standard bore and shaft fitting system. Shaft-hub connections are manufactured using this system as standard. They promise the best durability.
Features of the standard shaft
The tolerances for a shaft are marked with lower case letters. It specifies the tolerance zone within which the actual dimensions of the shaft must lie. As previously described, shafts are less frequently subjected to the unit system. Exceptions are shafts that are particularly long and expensive to manufacture. For these, the tolerance shift into the bore may be justified.
The unit bore
As with the shaft, DIN 7154 also specifies the dimensions of the ISO fits for the bore. They are marked with capital letters. In most cases, the bore is the preferred side manufactured according to the standardized fitting system. If the fit is insufficient, the shaft can be brought to the required dimension by simply turning it. This is less easy with a bore in a large, unwieldy or heavy component. Above all, the drilling tools used are much less flexible than the freely positionable chisel on a lathe. The tools used to drill holes are therefore more rigid and fix the diameter more firmly than a lathe does with the shaft.
Preferred fit types
The preferred type of fit for the unit bore and unit shaft system is the clearance fit. The clearance fit ensures simple installation and easy rotation from shaft to bore. However, transition and press fits are also possible. However, movement between the shaft and bore is undesirable and not recommended. There is a risk of mutual destruction.
Typical fit types with unit bore
In this application, the following types of fit with a unit bore are typical:
- H8/d9: The shaft can be easily mounted in the bore and has a noticeable clearance. This solution is good for applications where the shaft and bore are frequently separated and only low rotational speeds occur.
- H8/e8: Installation is also easy, but the play is less noticeable. This fit is suitable for permanent but easily twisted installation situations with a limited radius. An example of this are the pivots and bushes of the suspension of a room door.
- H8/f7: The shaft and bore are still easy to fit and there is still a noticeable amount of play. This fit only allows low rotational speeds. However, it is suitable for permanent assembly with complete rotations. An example of this is the axle-hub connection on inexpensive bicycles.
- H8/h9: The clearance is significantly reduced. This fit is suitable for installation situations in which the components are heated. The fit narrows considerably, so that a sliding fit can occur during operation.
- H7/g6 The play is barely noticeable. This fit is common in engine construction. It offers a firm hold and low wear. However, cooling and lubrication must be precisely maintained. The standard bore H7 is frequently found in general mechanical engineering.
- H7/h6: Some force is required for mounting. This pairing should always remain cool when moving. It offers hardly any scope for thermal expansion.
Economical production with the standard bore
The standard bore H7 and others allow economical and distributed production. The preferred shift of the tolerance zone into the shaft avoids costly reworking of the bore. In addition to the ideal fit, the combination of materials is also decisive for the durability of the connection. Even with the loosest clearance fit, the shaft and bore should therefore be made of different materials. This prevents galling, abrasion and mutual destruction. An ideal combination is steel and bronze, for example. Bronze is softer and has self-lubricating properties.
