A hydraulic damping approach to variable stroke tree shaker design

TR Number



Journal Title

Journal ISSN

Volume Title


Virginia Polytechnic Institute


Much interest in mechanically harvesting apples has been generated, but no completely successful system has yet been developed. Bruising of the fruit has been a prime drawback to current mechanical harvesting methods.

One practical approach to mechanized apple harvesting is to shake the trunk or limb of the tree to remove the fruit. With this method, apple detachment appears to be more dependent on length of stroke rather than shaking frequency. Therefore, a variable stroke shaker appeared to be a reasonable approach to a controlled rate of detachment from the tree.

The objective of this project was to develop a variable stroke tree shaking mechanism. The variable stroke feature was attained with the use of a hydraulic cylinder and metering valve in the boom of the shaker with the cylinder acting as a variable rate dashpot. The design was tested in the laboratory under varying conditions of effective limb mass and shaking frequency. For comparison, a constant stroke shaker was subjected to similar tests.

Both shaking mechanisms were subjected to a theoretical analysis. For the constant stroke shaker, the theoretical and test displacements of the limb were in good agreement, but the theoretical and test limb varied widely.

In the variable stroke shaker, the coefficient of damping of the cylinder was determined from test work and checked, using the differential equations for the system.

The testing performed in this investigation was limited to the laboratory, but for these conditions an effective variable stroke mechanism was developed.