Trapezoidal thread profile
As a key thread type for transmitting power and motion in mechanical transmission, the thread profile design of trapezoidal threads directly determines transmission efficiency, load-bearing capacity, and service life. The thread profile of a trapezoidal thread is an isosceles trapezoid with a standard thread angle of 30°. This symmetrical structure ensures that the thread is evenly stressed when subjected to axial force, reducing unilateral wear. Compared with triangular threads, the top and bottom of the threads of a trapezoidal thread are both flat, and the inclination angle of the tooth side is small. This not only reduces the friction coefficient of the thread pair, but also increases the contact area of the thread profile, enabling it to withstand greater axial loads. Therefore, it is widely used in heavy-duty transmission scenarios such as machine tool screws, hydraulic jacks, and presses.
The components of a trapezoidal thread profile include the crest, root, flank, profile angle, crest width, root width, and height. The geometric parameters of each element must strictly comply with the national standard ( GB/T 5796-2005). The crest is the top surface of the thread tooth. The crest width of an external thread is equal to the root width of a internal thread, and the crest width of an internal thread is equal to the root width of the external thread. This design ensures a reasonable radial clearance during thread engagement and avoids interference. The flank is the inclined surface connecting the crest and root. The angle between the two flanks is the profile angle (30°). The straightness error of the flank must be controlled within 0.01mm/100mm. Otherwise, it will lead to poor contact during thread engagement and cause localized stress concentration. The root of the thread adopts a fillet transition. The radius of the root of the external thread is 0.125P (P is the pitch) and the radius of the root of the internal thread is 0.25P. This fillet design effectively reduces stress concentration at the root and improves the fatigue strength of the thread.
The profile accuracy of trapezoidal threads significantly impacts transmission performance, primarily through three factors: profile angle error, flank straightness, and surface roughness. Profile angle error results in uneven force on both sides of the thread during engagement. Errors exceeding ±30° can exacerbate wear on one side and reduce thread life. Therefore, the profile angle error of high-precision trapezoidal threads must be controlled within ±10°. Excessive flank straightness error reduces thread contact area and increases local pressure. For applications requiring high transmission accuracy (such as precision machine tool lead screws), the straightness error should be ≤0.005mm/100mm. Regarding surface roughness, the Ra value of the thread flank must be ≤1.6μm. Excessive roughness increases the friction coefficient, reduces transmission efficiency, and accelerates thread wear. The thread surface after fine turning or grinding requires polishing to further reduce the surface roughness to below Ra0.8μm.
The CNC machining method for trapezoidal thread profiles depends on the required precision and production batch size. Common methods include turning, milling, and grinding. CNC Machining is suitable for single-piece, small-batch production. The profile is formed through the feed motion of the trapezoidal thread turning tool. During rough turning, a layered cutting method can be used to remove most of the excess stock. During finish turning, a forming cutter is used to ensure profile accuracy. Milling is suitable for medium-volume production. Disc or comb milling cutters are used to produce the thread through a spiral feed motion, achieving a 3-5 times higher efficiency than turning. Grinding is suitable for high-precision threads (Grade 5 and above). Using single- or multi-thread grinding wheels, the profile error can be controlled to within 0.005mm, with a surface roughness of up to Ra0.4μm. It is often used for the final CNC machining of precision lead screws. Regardless of the method used, symmetry of the profile must be maintained during processing. This can be verified in real time using a tool microscope or a dedicated gauge.
Trapezoidal thread profile inspection requires specialized measuring tools and instruments to ensure that all parameters meet design requirements. For standard-precision threads, a trapezoidal thread gauge can be used for comprehensive inspection. Acceptance is determined by smooth engagement of the go gauge and no-go gauges exceeding 1.5 turns. For high-precision threads, individual measuring tools are used, such as a universal protractor to measure the thread angle, a dial indicator and a standard template to measure flank straightness, and a roughness tester to check surface roughness. In mass production, an image measuring instrument can be used for fully automated thread profile inspection. This instrument can provide multiple parameters, including thread angle, crest width, and root width, in a single measurement, with an inspection rate of 30-50 pieces per hour. During inspection, the influence of ambient temperature must be considered. A constant temperature of 20±2°C minimizes measurement errors caused by thermal expansion and contraction, ensuring accurate inspection results. Rigorous inspection ensures the consistency and reliability of trapezoidal thread profiles, safeguarding the stable operation of mechanical transmission systems.
