Tolerances for trapezoidal threads
Trapezoidal thread tolerances are crucial technical indicators for ensuring connection and transmission accuracy. These encompass the allowable deviations of parameters such as the major, pitch, and minor diameters of the thread, as well as the thread profile angle and pitch. According to the GB/T 5796.4-2005 standard, trapezoidal thread tolerances are classified into seven grades: 3, 4, 5, 6, 7, 8, and 9, with Grade 3 being the highest accuracy and Grade 9 the lowest. Grades 7 and 8 are commonly used. Trapezoidal threads are categorized by application into transmission and connection applications. Transmission trapezoidal threads require strict control of the pitch diameter tolerance and cumulative pitch error to ensure transmission accuracy and efficiency. Connection trapezoidal threads primarily control the major and minor diameter tolerances to ensure connection reliability. Practice at a machine tool manufacturer has shown that each grade reduction in the pitch diameter tolerance of a trapezoidal thread screw can improve transmission accuracy by 20%-30%, but this also increases processing costs.
The pitch diameter tolerance of trapezoidal threads is key to controlling the fit. The pitch diameter is the primary fitting dimension of trapezoidal threads, and the position of its tolerance zone is determined by the basic deviation. The basic deviations for internal threads are H and h, while those for external threads are h, e, and c. A common fit is Tr40×7-7H/7e, where H indicates a zero basic deviation for the internal pitch diameter and e indicates a negative basic deviation for the external pitch diameter. The pitch diameter tolerance increases with increasing thread diameter and pitch. For example, for a trapezoidal thread with a nominal diameter of 30mm and a pitch of 6mm, the pitch diameter tolerance for internal threads with a grade 7 precision is 0.2mm, while for external threads it is 0.17mm. For internal threads with a grade 8 precision, the pitch diameter tolerance is 0.315mm, while for external threads it is 0.26mm. The tolerance grade of the middle diameter of the trapezoidal thread used for transmission is generally not lower than grade 7 to ensure the smoothness of transmission. The trapezoidal thread screws produced by a precision screw factory have a middle diameter tolerance controlled at grade 7, and the cumulative pitch error does not exceed 0.02mm/300mm, which meets the feed requirements of precision machine tools.
The tolerances for the major and minor diameters of trapezoidal threads primarily affect thread strength and assembly. The major diameter is the nominal diameter of the thread. The tolerance grade for the major diameter of external threads is the same as the pitch diameter, while the tolerance for the major diameter of internal threads is relatively loose. The minor diameter is the minimum diameter of the thread, and its tolerance is generally 1.5-2 times the pitch diameter tolerance. For trapezoidal threads subjected to axial loads, the dimensional accuracy of the major and minor diameters must ensure the effective thread height to avoid premature failure due to insufficient thread height. For example, for a Tr50×10-8H/8e thread, the tolerance for the major diameter of the internal thread is 0.5mm and the tolerance for the minor diameter is 0.63mm; the tolerance for the major diameter of the external thread is 0.4mm and the tolerance for the minor diameter is 0.5mm. A hydraulic equipment manufacturer, when manufacturing trapezoidal thread piston rods for hydraulic cylinders, ensured a reliable connection with the internal threads of the cylinder barrel and prevented oil leakage by controlling the major diameter tolerance of the external thread to within 0.1mm.
The pitch tolerance and profile angle tolerance of trapezoidal threads significantly impact transmission accuracy. The pitch tolerance controls the axial uniformity of the thread. Grade 7 has a pitch tolerance of ±0.025mm (pitch ≤ 6mm), and Grade 8 has a tolerance of ±0.05mm. The cumulative pitch error controls the total pitch deviation over a given length. Within a 300mm length, Grade 7 has a tolerance of 0.05mm, and Grade 8 has a tolerance of 0.1mm. The profile angle tolerance controls the symmetry of the thread profile. Grade 7 has a tolerance of ±15′, and Grade 8 has a tolerance of ±30′. Excessive deviation in the profile angle can lead to uneven force during thread engagement and increased wear. When a rolling mill manufacturer processes trapezoidal threads for roller adjustment, it controls the cumulative pitch error within 0.03mm/300mm and the tooth profile half-angle deviation within ±10′ through precision grinding, so that the roller adjustment accuracy reaches 0.01mm, meeting the thickness control requirements of plate and strip materials.
The selection of tolerances for trapezoidal threads requires a comprehensive consideration of both application requirements and processing costs. For precision transmission mechanisms, such as feed screws in CNC machine tools, grade 7 or 6 accuracy should be selected, with a H/e or H/c fit for the mid-diameter to ensure minimal clearance and high transmission accuracy. For general transmission mechanisms, such as screw pairs in lifting equipment, grade 8 or 9 accuracy can be selected, with an H/e fit to reduce processing costs. For connecting trapezoidal threads, such as flange connections in pressure vessels, only the major and minor diameter tolerances need to be controlled; the mid-diameter tolerance can be appropriately relaxed. Tolerance testing of trapezoidal threads requires specialized measuring tools. The mid-diameter can be measured with a trapezoidal thread micrometer or the three-needle measurement method, the pitch can be checked with a pitch gauge or a universal tool microscope, and the thread half-angle can be verified using a thread angle template or projector. Statistics from a metrology and testing agency indicate that trapezoidal threads produced strictly to tolerance standards have an average service life exceeding 50% compared to threads produced outside of tolerance. With the advancement of manufacturing technology, the tolerance control of trapezoidal threads is developing towards higher precision and stricter requirements, providing a strong guarantee for the precision transmission of high-end equipment.
