What is the H7/h6 fit clearance?
3 Answers
H7/h6 is a basic hole system fit clearance. H7/h6 is the tolerance fit code for mechanical components in the industry. H7 refers to the tolerance zone code of the hole to be matched, while h6 represents the tolerance zone code of the outer surface of the bearing (if the bearing is matched with the hole, then the bearing acts as the shaft). Below is more information about the clearance: 1. Function: The clearance serves to store lubricating oil, compensate for various errors, etc. Its size affects the relative motion between the hole and the shaft. 2. Main application: Clearance fits are primarily used for movable connections between holes and shafts, such as the connection between sliding bearings and shafts. The fit tolerance refers to the sum of the tolerance zones of the shaft and hole that form the fit, representing the allowable variation in clearance or interference. Its magnitude indicates the range of variation in the tightness of the fit and is an important comprehensive indicator for evaluating the quality of the fit.
This is an interesting question, and I often encounter beginners asking about it. The H7/h6 fit clearance is a standard for hole and shaft dimensions. Simply put, the hole tolerance is called H7, and the shaft tolerance is called h6. When combined, they indicate a certain gap between them, allowing the shaft to slide freely within the hole, but the gap is neither too large nor too small—just right. During design, the size range of the H7 hole is slightly larger; for example, the minimum hole diameter is a certain value, while the maximum is slightly wider. The h6 shaft dimensions are slightly smaller, with both the minimum and maximum shaft diameters being narrower. Thus, the maximum clearance is the maximum hole size minus the minimum shaft size, which might be tens of microns. The minimum clearance is the minimum hole size minus the maximum shaft size, which can approach zero but never negative. In automobiles, this is used in areas requiring flexible movement, such as connecting rod bearings or brake caliper pins, because too small a gap can cause seizing and overheating, while too large a gap can lead to noise, rattling, or part loosening. The standard setup ensures lubrication and longevity, and measuring dimensions with calipers during repairs is crucial. Paying attention to the tolerance system can help diagnose many minor issues.
In my work, I often deal with this type of fit. H7/h6 means the hole tolerance zone is at H7 grade, and the shaft is at h6 grade, which belongs to a clearance fit. The hole dimensions are generally larger, for example, the upper limit is more relaxed; the shaft dimensions are smaller, with a tighter lower limit, ensuring there is always a gap after assembly. The clearance can be calculated: the maximum clearance is the maximum hole size minus the minimum shaft size, typically between 0.01 to 0.05 millimeters; the minimum clearance is the minimum hole size minus the maximum shaft size, which may be very small but does not interfere. Applied to automotive parts, such as crankshafts or wheel hub bearings, this type of fit is commonly used to ensure smooth engine operation and prevent seizing due to thermal expansion. Choosing the wrong fit type, such as switching to a transition fit, could make the shaft difficult to insert, leading to assembly failure or premature wear. During maintenance, always check according to the manual specifications and avoid arbitrarily changing tolerance settings. This not only extends component lifespan but also reduces the frequency of repairs.
