Mobile cranes perform a wide variety of tasks, typically of the heavy-duty kind. The work they do and the locations at which they operate are frequently exposed to harsh climatic conditions in places with insufficient infrastructure. This means that the sites at which the cranes are positioned and the environment in which they move is often not entirely suitable for this kind of heavy construction equipment. Accordingly, there are high loads acting on the components, which often wear out prematurely as a result. A new sealing solution for swivel joints in cranes subjected to high loads, which combines a polyurethane O-ring with a nobrox® backup ring, has effectively remedied this issue.
Swivel joints are generally used for swiveling and/or rotating hydraulic connectors. The type of ball bearing mounted swivel joint from Parker Ermeto, which specializes in a wide range of industrial components, is unique in this form and features a particularly robust design. In addition to applications in booms or cranes, these swivel joints can essentially be installed in excavators, drill units or diverse stationary applications as well.
Aside from a ball bearing mounted race made of bearing steel, which transmits rotary movements even under extremely high interior pressure loads of up to 420 bar with nearly no losses, such high-performance swivel joints require a seal that encloses the hydraulic fluid while permitting relative rotary movements in its seat under maximum contact pressure without abrasion or extrusion. Drag-in of the hydraulic fluid into the ball guide must be prevented – even after an extremely cold night when the mobile crane after an eight-hour period of rest with unheated units is set up, i.e. started up again. Furthermore, a sealing ring as atmospheric protection is provided against external dirt and drag-in of dust or condensate.
The ball bearing mounted swivel joints utilized in mobile cranes posed the problem that the NBR O-rings and back-up rings used for sealing could extrude into the gap between the ball bearing mounted pivot and the surrounding housing after a relatively short period of time. Consequently, the backup ring was practically pulverized, resulting in wear of the O-ring. The extremely high contact pressures and the simultaneous slow rotating movement were identified as the causes. As a result, the sealing elements were pushed into the gaps induced by the manufacturing process, which ultimately led to seal wear. This effect can be observed during the controlled lowering of the jib under high operating pressures and simultaneous occurrence of extreme flow velocities, i.e. above those established by the engineering design criteria.
Thanks to its excellent wear resistance and good anti-frictional properties, Parker Prädifa’s new thermoplastic sealing material nobrox® (PK) was taken into consideration as a material for the backup ring at an early stage. Another objective was to enhance the robustness of the O-ring as well by utilizing a material with higher wear resistance. Therefore, instead of the previously used NBR material, an Ultrathan® (TPU) compound from Parker Prädifa’s portfolio was selected.
In the physical laboratory, the swivel joint with the new sealing set was tested in a wide range of extreme operating conditions exceeding those of the application and simulating the operating parameters actually prevailing on the vehicle with maximum realism. These tests run over 10,000 cycles were successfully passed, revealing that the maintenance-free operating period of the swivel joint far exceeds the typical frequency and operation sequences of a jib.
Article contributed by
Michael Dillmann, account manager,
Engineered Materials Group Europe
Carsten Schippers, research & development,
Fluid Connectors Group, Tube Fittings Division Europe