1Nominal size and actual size
A、Nominal size: it is the nominal size regulated in the standard, and is the ideal size expected by the user and manufacturer, and is also the ordered size indicated in the contract.
B、Actual size: it is the actual size obtained during production, and this size is usually larger or smaller than the nominal size. The phenomena are called deviation.
②Deviation and tolerance
A、Deviation: during production, as the actual size is hard to achieve the requirements of nominal size, ie. the actual size is often larger or smaller than nominal size, an allowable difference between actual size and nominal size. The positive difference is called positive deviation, while the negative difference is called negative deviation.
B、Tolerance: the sum of the absolute values of positive deviation and negative deviation regulated in the standard is called tolerance, also called “tolerance zone”.
Delivery length is also called user required length or contract length. In the standard, there are several regulations on the delivery length in the standard, as following:
A、Common length (also called random length): the length within the length range regulated in standard and without fixed length requirements is called common length. For example, it is regulated in the structural tube standard that: the common length of hot rolled (extruded, expanded) steel tube is 3000 mm -12000mm; while the common length of cold-drawn (rolled) steel tube is 2000 mm-10500mm.
B、Cut length: cut length is often within the range of common length, and is the certain fixed length size required in contract. However, it is impossible to cut out absolute cut length always in the actual operation, thus allowable positive deviation of cut length is regulated in the standard.
Take structural tube as an example:
The finished product rate of cut-to-length tube is greatly lower than the common-length tube, thus the price increasing request brought out by the manufacturer is reasonable. The price increasing rates of each enterprise are not consistent; generally, the price can be increased by 10% on the basis of basic prices.
C、Double length: double length should be within the range of common length usually, the individual double length and the multiple to compose the total length should be indicated in the contract (for example, 3000 mm × 3, that is triple of 3000 mm, with a total length of 9000mm). In the actual operation, an allowable positive deviation of 20mm should be added to the total length, as well as plus the cutting margin of each double length. Take structural tube as an example, the required cutting margin is 5 - 10mm for the steel tube with a diameter ≤159mm; 10-15mm for the steel tube with a diameter ＞159mm.
If there are no regulations in the standard, the double length deviation and cutting margin should be negotiated by both supplier and buyer and indicated in the contract. The same as cut length, double length can reduce the finished product rate of the enterprise greatly, thus the price increasing request brought out by the manufacturer is reasonable, and the price increasing rate is essentially the same as the price increasing rate of cut length.
D、Range length: range length is usually within common length range; in the case that the user requires the length in a fixed length range, it should be indicated in the contract. For example: that of common length is 3,000-12000mm, while that of cut length is 6000-8000mm or 8000 ~ 10000mm.
It may be seen that, the requirements on range length is easier than the cut length and double length, but stricter than the common length a lot, and it may reduce the finished product rate of enterprises. Therefore, the price increasing request brought out by the manufacturer is reasonable; generally, the price can be increased by about 4% on the basis of basic prices.
④Uneven wall thickness
Steel tube wall thickness is impossible to be the same, uneven wall thicknesses may exist on the cross-section and longitudinal tube objectively, ie. uneven thickness. To control this uneven phenomenon, the allowable indexes of uneven thicin the steel tube standard; generally, it is regulated not to exceed 80% of the tolerance of wall thickness (which should be imkness are regulated plemented after negotiating between the supply and the buyer).
The external diameter of the cross section of round steel tube may be uneven, that is the maximum external diameter and the minimum external diameter may be not perpendicular to each other, the difference between the maximum external diameter and the minimum external diameter is ellipticity (or non-round degree). In order to control the ellipticity, the allowable indexes of ellipticity are regulated in some steel tube standard; generally, it is regulated not to exceed 80% of the tolerance of external diameter (which should be implemented after negotiating between the supply and the buyer).
The steel tube is curvilinear along the length direction, and the bending degree indicated with figures is called curvature. The curvature regulated in the standard can be divided into two categories as follows:
A、Local curvature: 1-meter long ruler can be used to measure the chord height (mm) at the maximum bending location, ie. local curvature value, its unit is mm/m, for example: 2.5 mm / m. The method is also applied to the curvature of tube end.
B、Overall curvature of the total length: tighten a cord at both sided of the tube to measure the maximum chord height (mm) of the bending location of the steel tube, and then convert it into the percentage of length (m), that is the overall curvature along the length direction of the steel tube.
Example: the length of steel tube is 8m, and the maximum chord height is measured as 30mm, thus the overall curvature of the tube should be:
Size exceed can also be called as allowable deviation of the size exceeding the standard. Here the "size" mainly refers to the external diameter and wall thickness of steel tube. Usually, someone calls the size exceed as “tolerance exceed”, but this way of equate deviation to tolerance is not rigorous, and it should be called as "deviation exceed". Here the deviation may be "positive" or "negative", the "positive" deviation and "negative" deviation hardly exceed the standard simultaneously in the same batch of steel tube.