Functions and Requirements of the Horizontal Slip Table
Many products require vibration tests in the x, y, and z axes. For small test specimens, changing the fixture’s installation state or using a fixture inversion method can achieve vibration in three directions on a vertical vibration table. However, for large test specimens, the fixture inversion method may cause excessive eccentric torque. Some special products (such as gyroscopes) require that the installation state of the main axis relative to the Earth’s position remains unchanged during vibration tests. Therefore, neither changing the fixture’s installation state nor using the fixture inversion method can achieve tri-axial vibration tests. In such cases, a horizontal slip table is required to perform vibration tests in all three directions. The functions of a horizontal slip table are:
- To act as a load-bearing platform for vibration tests, supporting the static load of large and medium-sized test specimens.
- To transmit the motion of the vibration generator to the test specimen in the horizontal directions (x, y).
To complete the above two functions, the performance requirements for the horizontal slide table are as follows:
- High load-bearing capacity and efficient force transmission rate.
- Light weight but with a high first-order resonance frequency.
- High resistance to rolling and pitching.
- Low waveform distortion on the working surface.
- Good uniformity of acceleration at various points on the working surface.
- Minimal lateral movement of the working surface.
Structure and Types of Horizontal Slip Tables
Previously used horizontal slip tables in vibration tests include mechanical slip tables, air spring slip tables, hydrostatic bearing slip tables, oil film slip tables, and hydrostatic guideway slip tables. Currently, the most commonly used are the “oil film slip table” and the “oil film combined with hydrostatic guideway slip table.” The former is suitable for small and medium-sized horizontal slip tables, while the latter is suitable for large and medium-sized horizontal slip tables requiring high load-bearing capacity.
- Oil Film Slip Table
This is currently the most widely used horizontal slip table paired with electro-dynamic vibration tables and hydraulic tables. The main components of a plane oil film slip table include an aluminum slip plate (working surface), a granite guide platform, the table body, an oil source, a slip plate guiding mechanism, and connectors between the slip plate and vibration table (commonly known as the “bullhead”).
- Hydrostatic Bearing Slip Table
The oil film slip table is a widely used horizontal slip table, but its upper working frequency limit is constrained by the length of the slip plate—the longer the slip plate, the lower the upper working frequency—often failing to meet the requirements of random vibration tests (with upper working frequencies above 2kHz). Another issue is the poor resistance to pitching and rolling, especially when the test specimen is heavy and the vibration acceleration is high, or when the specimen installation is eccentric, causing the slip plate and base to separate, rendering the slip table inoperative. To improve the slip table’s resistance to pitching and rolling and enhance the slip plate’s dynamic characteristics, the hydrostatic bearing slip table is a viable structural option.
A hydrostatic bearing horizontal slip table consists of multiple hydrostatic bearings between the slip plate and the base. Each hydrostatic bearing comprises a bearing sleeve (bearing shell) and a slip column (main shaft). The upper end of the bearing sleeve is a highly flat plate (slip block). The upper surfaces of all slip blocks are adjusted to be level with the lower surface of the slip plate, and the slip plate (working surface of the horizontal slip table) is fixed to the slip blocks. The slip columns are supported on the bearing frame, which is fixed to the support plate (base) of the slip table. Unlike general hydrostatic bearings, the main shaft remains stationary, while high-pressure oil is pumped into the hydrostatic bearings, causing all the slip blocks to float on the bearing frame, allowing the slip plate to reciprocate on the bearing frame. The hydrostatic bearing assembly, installed beneath the slip plate, typically centers around the geometric centerline of the slip plate, with at least two rows of assemblies symmetrically installed on either side. Each row of bearing assemblies has 2-4 bearing frames, with more hydrostatic bearing assemblies added as needed for load-bearing. For a hydrostatic bearing horizontal slip table, alignment between the slip table and the vibration table is crucial; otherwise, it can add a lateral load to the slip table, causing bumping and twisting of the slip plate and preloading the hydrostatic bearings, reducing their load-bearing capacity. This type of slip table boasts high technical performance indicators such as high load-bearing capacity, low friction damping, strong resistance to pitching and rolling, and excellent guiding performance. However, it also features a complex structure, high manufacturing process requirements, difficult assembly, adjustment and maintenance, and requires a high-pressure oil source, making it suitable only for large-size, heavy-load, large-displacement, and high-acceleration vibration tests.
- Combination Slip Table of Hydrostatic Bearing and Oil Film
This structure combines 2-4 hydrostatic bearings on the central axis of the granite guide platform of an oil film slip table, with the bearing sleeves bolted to the underside of the slip plate and the bearing frames fixed to the support plate of the oil film slip table. Using a “multi-oil-chamber supply” oil film slip table scheme, this type of slip table also has strong anti-offset load capabilities. Additionally, the hydrostatic bearings restrict the lateral movement and torsion of the slip plate, as well as its vertical movement, thereby enhancing the slip table’s resistance to rolling and pitching moments.
This type of slip table combines the benefits of both oil film slip tables and hydrostatic bearings: high slip plate stiffness, strong load-bearing capacity, excellent resistance to pitching and rolling, elimination of guiding and limiting mechanisms, and relatively straightforward manufacturing processes, making it a highly preferred horizontal slip table structure.