Deformation of A Barrel Hole Due to Preloads and Operating Loads and its Effect on Spool Fit Size
In a hydraulic manifold block for construction machinery, spool valves are used to control the movement of the machine. Spool valves have two elements: a cylindrical barrel hole (1) in which a spool (2) slides. By moving the spool in the barrel hole, apertures open and close which enables the control of the connected hydraulics. In order to achieve the best control and stability and to avoid internal leakage of the valve in operation, a minimal radial clearance and a fine surface finish between the spool and the barrel is required. At the same time, some clearance is required for lubrication between the valve spool and the barrel hole to prevent friction that would prematurely age the components or cause the spool to seize.
Consequently, an optimum must be found between the conflicting requirements.
Unfortunately, the clearance between spool and barrel is not constant: while the parts will always have a known deviation in size and form, depending on manufacturing accuracy, it is also affected by deformation caused by external forces on the manifold, as well as the hydraulic pressure acting on the structure. In summary, the following factors influence the fit of the spool in the barrel:
1: Barrel hole form and size deviations due to the machining process.
2: Barrel hole deformation due to preloads, such as bolt pretension
3: Barrel hole deformation due to operating loads, such as internal hydraulic pressure
To find the necessary fit between the barrel hole and the spool all three points must be taken into account. In order to gain deeper understanding of the influences, a simulation using COMSOL Multiphysics® was carried out.
A model of a typical barrel hole of a hydraulic control valve is created in 3D CAD and imported into COMSOL Multiphysics®. For different bolt pretension and pressure loads the deformation is calculated. Subsequent post-processing in COMSOL Multiphysics® yields the maximum valve spool diameter without interference.
Knowing the diameter of the largest fitting spool and the achievable machining tolerances, the manufacturing tolerances can be set in a way that barrel hole and spool have the best possible clearance, enough to have sufficient lubrication and avoid jamming, but in a way that has the least impact on the performance of the hydraulic system.
To confirm the calculations done in COMSOL Multiphysics® a demonstrator will be fabricated and tested, the results then will be compared with the results from COMSOL Multiphysics® .
