Hydraulic Cylinder Vibration - How to Remedy?

[AlliedAuto]

Hi all,

I work for a machine building company. We've built a hydraulic lifting machine for lifting a product (long copper bars weighing up to 200kg). The setup is as follows:

1.There are two carriages on either end of a mild steel frame. One of the carriages is fixed and the other is sliding (attached to a Festo linear axis). The sliding carriage can traverse along the x-axis meaning it can come closer to the fixed carriage or move away from it depending on the length of the product in the nests.

2.There are hydraulic cylinders (2 on the fixed carriage and 2 on the sliding carriage) that allow a nest, for retaining the product, to move up or down, over or accross. The powerpack that powers these cylinders has two geared flow dividers which keep the cyliders operating at the same speeds. If they are not in travelling at the same speed the product would not be safe in the nests and might fall off, hence the need for the geared flow dividers.

We're having significant vibration problems when the vertical cylinder (which moves the nest up and down) extends, i.e. moves the nest down. It starts to vibrate very badly but it's only happening on one end of the machine (fixed end).

We've done loads of tests including:

-Changed out the seals one of the cylinders
-bled the system to remove air
-swapped cylinders around to opposite ends to make sure it's not a meachanical problem
-changed the setup of the geared flow dividers
-switched around the pipes (the powerpack is not centred bewtween the two carriages so there are longer hoses going to the sliding carriage and shorter hoses to the fixed carriage)

All have resulted in the same thing - vibration when the vertical cylinder drops the nest down. There is considerable weight on it. When we have taken the weight off the cylinder, i.e. left the cylinder free to extend without having anything connected to it there is no vibration.

I've attached a few pictures of the setup of the cylinders on the machine. Please see the links.

http://www.box.net/shared/tdrqut813p

http://www.box.net/shared/j28jyv1tl7

http://www.box.net/shared/u6fr0be2f5

please note that hydraulic hosing and is not in the pictures and the powerpack isn't an exact model (doesn't show geared flow dividers)

Any help appreciated.

 

[hydtools]

Do you have a hydraulic schematic you can post?

Ted

 

[desertfox]

Hi AlliedAuto

I couldn't tell from the pictures you have posted but I was wondering if the load you place on the nest was creating an offset load on the cylinder ie causing the cylinder rod to try and bend, which in turn creates binding within the cylinder.

Regards

desertfox

 

[romke]

given the fact that the problem only occurs at the fixed site, could indicate that there is a stiffness problem in the frame. can it be that the fixed carriage is deforming somewhat when you lower the load, whereas the movable support can adjust itself somewhat to "align" itself properly? if that is the case it might lead to stick-slip conditions in the "fixed" cylinder, in turn leading to vibration.

from the experiments you already have carried out it seems to me that it is not a mechanical fault in the cylinders or the hydraulics.

 

[Newmanite]

You may have found the natural frequency of the mounting point of the ram to the rest of the table?Can you fit a piece of absorbant material between the ram fixed end and the mount?Try some rubber or bake alite type material cheap and inexpensive.The back wall of the fixture is that thinner than the mount for the ram?If so this could give you harmonic problems also.