Abstract
The complex structure of the steering gear rocker shaft, particularly the low precision and efficiency of manual positioning and clamping during the gear shaping process of its sector teeth. To overcome these issues, a reasonable positioning reference was selected, and a dedicated fixture designed for use on a numerical control gear shaper machine. The fixture employs a spring collet centering clamping mechanism powered by a hydraulic cylinder, thereby enhancing production efficiency and automation levels. By swapping out the spring collet and cone sleeve, it can accommodate various rocker shaft clamping requirements, enabling group machining.
1. Introduction
The rocker shaft serves as a core component in hydraulic power recirculating ball steering gears. The front and rear shafts, subject to significant loads, necessitate reliable sealing. The fan-shaped teeth located in the middle section, requiring high precision, undergo gear shaping after tooth milling. Manual positioning and clamping are not only inefficient and challenging for fixture design but also difficult to ensure processing accuracy. Therefore, designing a reasonable process and dedicated fixture, utilizing a hydraulic cylinder as the power source and an automatic centering clamping mechanism, can guarantee the quality and production efficiency of the gear shaping process, enhancing the level of machining automation.
2. Process Analysis of Gear Shaping for Rocker Shaft
2.1 Process Requirements
The rocker shaft mainly comprises front and rear shafts of varying lengths and fan-shaped teeth. The long shaft end is also machined with a spline for connection with the vertical arm of the steering system.
The tooth tip circle of the fan-shaped teeth forms a cone, and the fan-shaped tooth cross-section obtained at any axial section is unique, resulting in different profile shift coefficients for each section. During gear shaping, the main movement direction of the gear shaper cutter forms a certain angle with the workpiece outer circle axis.
The rocker shaft material is 20CrMnTiH3. Taking a specific rocker shaft model as an example, the technical requirements of the fan-shaped teeth at the zero profile shift section.
The tooth tip radius R56.22 has a single-part variation not exceeding 0.025.
2.2 Process Analysis
The rocker shaft blank is a forging, typical of axial parts. The outer diameters of the two sections are mainly processed by turning and grinding, while the spline on the long shaft end is produced through hobbing. Due to the fan-shaped teeth not being directly die-forged into shape, to improve production efficiency and ensure processing accuracy, a forming milling cutter is first used for rough milling of the tooth fan, followed by gear shaping on a gear shaper. The processing sequence follows the principle of base surface precedence, starting with milling both end faces and drilling center holes, which serve as positioning references for subsequent operations. The two center holes are used for combined positioning with two centers, enhancing the rigidity of the workpiece after clamping. Subsequent operations include rough turning of the fan tooth side and top surfaces, short shaft outer diameter, and long shaft outer diameter; semi-finish turning of the short shaft outer diameter, drilling and boring holes on the short shaft end face, and threading; semi-finish turning of the long shaft outer diameter and tooth fan; rough milling of the tooth fan; semi-finish and finish gear shaping; hobbing of the involute spline; grinding the center holes after heat treatment; and finally, precision grinding of the long and short shaft outer diameters on an external cylindrical grinding machine.
The fan-shaped teeth have positional requirements with the outer diameters of the long and short shafts. During gear shaping, to ensure uniform machining allowances, the circumferential freedom of the workpiece must be restricted. Therefore, selecting a reasonable positioning reference is crucial for the gear shaping process.
3. Positioning Scheme Design
Positioning is the process of aligning a workpiece correctly on a machine tool or fixture before machining. The rationality of selecting the positioning reference is highly important. By analyzing the rocker shaft structure, it is found that the two sides of the die-forged tooth fan can reliably achieve angular positioning, and the tooth fan step face and long shaft outer diameter can also serve as positioning references. Therefore, it is decided to use the long shaft outer diameter to restrict four degrees of freedom, the tooth fan step face and side to restrict one degree of freedom each, totaling six degrees of freedom for complete positioning. Selecting the long shaft outer diameter to restrict four degrees of freedom follows the principle of coincident datums, ensuring the positional requirement between the outer diameter and tooth fan. The specific positioning scheme.
The zero profile shift 0-0 section of the tooth fan is 5 mm away from the step face, chosen as the axial positioning reference, also following the principle of coincident datums, thus eliminating positioning errors. Both the milling and gear shaping processes use the tooth fan side as the positioning reference to restrict angular freedom, adhering to the principle of uniform datum for higher positioning accuracy.
4. Design of the Dedicated Fixture
To accommodate different rocker shaft models, the components of the dedicated fixture for gear shaping should be designed in a series. By replacing a few components, group machining can be achieved, meeting the requirements of product variety changes, low cost, and high quality.
4.1 Spring Collet Design
Considering the structure of the numerical control gear shaper machine worktable and the positioning scheme, a spring collet centering clamping mechanism is adopted, relying on uniform elastic deformation of the centering clamping elements to achieve centering and clamping. When the spring collet moves relative to the cone sleeve, the spring petals contract radially, centering and clamping the long shaft outer diameter of the rocker shaft. Unlike standard spring collets, Tightened by a hydraulic cylinder, so its tail is designed with a structure to connect with the piston rod. A pin passes radially through the outer circle of the spring collet tail, connecting the spring collet and piston rod together and transmitting tensile force.
Different rocker shaft models have varying outer diameters, so replacing spring collets with different inner diameters can accommodate this. The spring collet suitable for the gear shaping process of a specific rocker shaft model, with a D1 dimension of φ51.1 mm.
4.2 Cone Sleeve Design
An essential component paired with the spring collet is the cone sleeve. After precision machining its inner cone surface, it should undergo final grinding with the outer cone surface of the spring collet to ensure their mating accuracy.
To ensure the mating accuracy between the cone sleeve inner hole and spring collet, the 15° cone hole should be inspected using a color contrast method, with the contact area exceeding 80%, and the coaxiality of the cone hole with datum A being 0.01.
4.3 Clamping Mechanism Design
The gear shaping process after tooth milling is completed in two passes: semi-finish gear shaping and finish gear shaping. The cutting parameters for these operations differ, with semi-finish gear shaping utilizing a cutting speed Vc of 204 m/min, a feed rate f of 0.33 mm/stroke, and a back cutting depth ap of 2.5 mm. For finish gear shaping, the parameters are adjusted to a cutting speed Vc of 245 m/min, a feed rate f of 0.28 mm/stroke, and a back cutting depth ap of 0.85 mm. Since the semi-finish gear shaping generates higher cutting forces due to the larger back cutting depth and feed rate, it is sufficient to calculate and verify the clamping force required for this operation.
To determine the clamping force, the unit cutting force kc is first calculated or obtained from relevant tables, which in this case is 1,962 N/mm². The nominal cross-sectional area AD of the cut is calculated as ap * f = 0.825 mm², taking into account the back cutting depth and feed rate. With various correction factors for different cutting conditions, the main cutting force Fc and the feed force Ff can be derived. For the given example, Fc is calculated to be 2,153 N, and Ff is 861 N.
The main cutting force Fc acts axially, while the feed force Ff acts tangentially, both of which are relevant for calculating the clamping force. The radial force, which points towards the center of the workpiece, does not need to be calculated or verified due to the structural characteristics of the spring collet centering clamping mechanism.
The clamping force required to prevent the workpiece from slipping under the action of cutting torque and axial cutting force is calculated using the formula:
F = (2 * K * f * Me) / (r * π)
where K is the safety factor (taken as 1.8), f is the friction coefficient between the elastic collet and the workpiece (taken as 0.15), Me is the torque generated by the feed force (Ff * ra, where ra is the radius of the tooth tip circle), r is the radius of the clamped outer circle of the workpiece. For the given example, with a gear modulus m of 8.5, a full-circle tooth number z of 11, and a tooth tip height ha of 6.8 mm, the tooth tip circle radius ra is 55.25 mm. Thus, the torque Me is 47.57 N·m. The clamped outer circle diameter of the rocker arm shaft, which serves as the positioning base, has been machined to φ55 mm, so the radius r is 27.5 mm. Applying these values to the formula, the calculated clamping force F is approximately 20,757 N.
Each petal of the spring collet exerts an elastic force Rc of 474 N on the shaft sleeve. The axial force Q required to clamp the workpiece is then calculated using the formula:
Q = (F + Rc) * tan(α + ρ1) + F * tanρ2
where ρ1 is the friction angle between the spring collet and the cone sleeve (taken as 11.5°), ρ2 is the friction angle between the spring collet and the workpiece (taken as 8.5°), and α is the half-cone angle of the spring collet (7.5°). Substituting the relevant parameters into the formula, the calculated axial force Q is approximately 10,412 N.
Therefore, when the hydraulic cylinder tightens the spring collet, it should provide an axial tensile force of approximately 10.41 kN. According to the机械设计手册, if the cylinder efficiency is taken as 0.8, a pull-rod hydraulic cylinder with a rated pressure of 3.5 MPa and a cylinder diameter of 80 mm can provide a maximum thrust of 17.94 kN, which meets the usage requirements.
In conclusion, the clamping mechanism for the special fixture for gear shaping of the steering gear rocker shaft is designed to provide sufficient clamping force to hold the workpiece securely during the cutting process. The use of a hydraulic cylinder as the power source simplifies the fixture design and ensures reliable centering and clamping of the workpiece, improving the automation level of machining.