①Nominal size and actual size
A. Nominal size: It is the nominal size specified in the standard, the ideal size expected by users and manufacturers, and the order size indicated in the contract.
B. Actual size: It is the actual size obtained in the production process, which is often larger or smaller than the nominal size. This phenomenon of being larger or smaller than the nominal size is called deviation.
② Deviation and tolerance
A. Deviation: In the production process, because the actual size is difficult to meet the nominal size requirements, that is, it is often larger or smaller than the nominal size, so the standard stipulates that there is a difference between the actual size and the nominal size. If the difference is positive, it is called positive deviation, and if the difference is negative, it is called negative deviation.
B. Tolerance: The sum of the absolute values ??of positive and negative deviation values ??specified in the standard is called tolerance, also called “tolerance zone”.
The deviation is directional, that is, expressed as “positive” or “negative”; the tolerance is not directional, so it is wrong to call the deviation value “positive tolerance” or “negative tolerance”.
③Delivery length
The delivery length is also called the length required by the user or the length of the contract. The standard has the following provisions on the delivery length:
A. Normal length (also known as non-fixed length): Any length within the length range specified by the standard and no fixed length requirement is called normal length. For example, the structural pipe standard stipulates: hot-rolled (extrusion, expansion) steel pipe 3000mm ~ 12000mm; cold drawn (rolled) steel pipe 2000mmmm ~ 10500mm.
B. Length of fixed length: The length of fixed length should be within the usual length range, which is a certain fixed length dimension required in the contract. However, it is impossible to cut out the absolute fixed length in actual operation, so the standard stipulates the allowable positive deviation value for the fixed length.
According to the structural pipe standard:
The yield of the production of fixed-length pipes is larger than that of ordinary length pipes, and it is reasonable for the manufacturer to ask for a price increase. The price increase varies from company to company, but it is generally about 10% higher than the base price.
C. Double ruler length: The multiple ruler length should be within the usual length range, and the single ruler length and the multiple of the total length should be indicated in the contract (for example, 3000mm×3, that is, 3 multiples of 3000mm, and the total length is 9000mm). In actual operation, the allowable positive deviation of 20mm should be added on the basis of the total length, and the incision allowance should be reserved for each single ruler length. Taking the structural pipe as an example, it is stipulated that the incision margin should be reserved: the outer diameter ≤ 159mm is 5 ~ 10mm; the outer diameter > 159mm is 10 ~ 15mm.
If the standard does not specify the length deviation of the double ruler and the cutting allowance, it should be negotiated by both parties and indicated in the contract. The double-length scale is the same as the fixed-length length, which will greatly reduce the yield of the manufacturer. Therefore, it is reasonable for the manufacturer to raise the price, and the price increase is basically the same as the fixed-length increase.
D. Range length: The range length is within the usual range. When the user requires a fixed range length, it should be indicated in the contract.
For example: the usual length is 3000~12000mm, and the range fixed length is 6000~8000mm or 8000~10000mm.
It can be seen that the range length is looser than the fixed-length and double-length length requirements, but it is much stricter than the usual length, which will also reduce the yield of the production enterprise. Therefore, it is reasonable for the manufacturer to raise the price, and the price increase is generally about 4% above the base price.
④ Uneven wall thickness
The wall thickness of the steel pipe cannot be the same everywhere, and there is an objective phenomenon of unequal wall thickness on its cross section and longitudinal pipe body, that is, the wall thickness is uneven. In order to control this unevenness, some steel pipe standards stipulate the allowable indicators of uneven wall thickness, which generally do not exceed 80% of the wall thickness tolerance (executed after negotiation between the supplier and the buyer).
⑤ Ovality
There is a phenomenon of unequal outer diameters on the cross section of the circular steel pipe, that is, there are the maximum outer diameter and the minimum outer diameter that are not necessarily perpendicular to each other, then the difference between the maximum outer diameter and the minimum outer diameter is the ovality (or not roundness). In order to control the ovality, some steel pipe standards stipulate the allowable index of ovality, which is generally specified as not exceeding 80% of the outer diameter tolerance (executed after negotiation between the supplier and the buyer).
⑥Bending degree
The steel pipe is curved in the length direction, and the curve degree is expressed by numbers, which is called the bending degree. The bending degree specified in the standard is generally divided into the following two types:
A. Local bending degree: measure the maximum bending position of the steel pipe with a one-meter-long ruler, and measure its chord height (mm), which is the local bending degree value, the unit is mm/m, and the expression method is 2.5 mm/m. . This method also applies to tube end curvature.
B. Total bending degree of the whole length: Use a thin rope to tighten from both ends of the pipe, measure the maximum chord height (mm) at the bend of the steel pipe, and then convert it into a percentage of the length (in meters), which is the length direction of the steel pipe full-length curvature.
For example, if the length of the steel pipe is 8m, and the measured maximum chord height is 30mm, the bending degree of the whole length of the pipe should be:0.03÷8m×100%=0.375%
⑦The size is out of tolerance
The size is out of tolerance or the size exceeds the allowable deviation of the standard. The “dimension” here mainly refers to the outer diameter and wall thickness of the steel pipe. Usually some people call the size out of tolerance “out of tolerance”. This kind of name that equates deviation with tolerance is not strict, and should be called “out of tolerance”. The deviation here may be “positive” or “negative”, and it is rare that both “positive and negative” deviations are out of line in the same batch of steel pipes.
Post time: Nov-14-2022