Understanding gas strut mounting points and positions.
There are two primary mounting points, the ‘fixed’ and ‘moving’ mounting points. As the names suggest, the fixed mounting point remains fixed, whereas the moving mounting point rotates through an arc as the application opens and closes.
As a rule of thumb when positioning, Camloc start with the moving mounting point approximately 1/3 the length of the lid from the hinge as shown in figure two below:
Typical mounting points example. This provides an extremely rough guide as to where to place a strut, but if this is developed further it will also give an indication of the size of the required product.
There are two differing mounting orientations available to the designer, these are ‘Push Up’ and ‘Flip Over’ mounting. In both cases, it should always be mounted “rod down” when in the fully closed position to ensure proper lubrication of the seal package.
As a general rule, Camloc’s preferred mounting is the ‘Push Up’ design orientation.
Push Up Design
This type of mounting can be identified by the fact that the end at the lowest point when closed, remains in its lowest position when fully open. It can also be identified by the moving mounting point being located closer to the hinge than the fixed mounting point.
Unless a means is utilised to lubricate the rod, then the strut should always be mounted rod down to ensure proper lubrication of the main seal. The main drawback of this mounting position is the additional strength required in the application being lifted, particularly the hinge; this is due to increased cantilever of the lid from the support provided by the strut.
The major advantage with this type of mounting and rod down orientation is that it provides consistent damping at the end of the stroke. This is due to oil always being at the bottom of the tube; thus, damping will always occur at the same point in the lift cycle.
This type of mounting can be identified by the end at the lowest point of the strut when closed, rotates to the highest point when fully open. It can also be identified by the moving mounting point being located further away from the hinge than the fixed mounting point
Unless a means is utilised to lubricate the rod then it is advised to mount rod down, ensuring proper lubrication of the main seal in the closed position.
The main drawback of this mounting position is the lack of damping control throughout the stroke. At the start of the stroke, the oil is at the bottom around the main seal. As the horizontal position is passed, the oil begins to run down the tube towards the tube end.
Along this point it will meet the piston moving up through the tube. When the piston meets the oil, extension will be slowed until the oil has passed through. At this point, the extension speed will increase and will reach the end of the stroke with no damping.
The advantage of this design is that it places less strain on the hinges than the push up design.
Mounting: Crossover, Self-Rise & Self-Close
Crossover is the point at which the strut takes over the lifting action (or gravity takes over to close). This will normally be around 10° to 30° from fully closed. In practice, this will vary by several degrees between opening and closing, due to factors such as friction of the internal components of the spring, hinges and end connectors.
Self-Rise & Self-Close
Self-rise is the angle at which the lid is lifted without any assistance from the operator. Similarly, self-close is the angle at which the lid will close without any assistance. In most cases, it is undesirable to have the lid open without any operator input (referred to as “instant lift”). This behaviour is unpredictable to the operator and can allow the lid to open without any warning, in instances where the P1 force increases above nominal due to elevated ambient temperatures, for example.
Effects of Temperature on Handling
Temperature not only affects the output force, but also the handling forces. The following graphs shows behaviour on an application at 20°C and then again at 65°C. As is seen in the figure below, at 20°C crossover occurs at an opening angle of around 7°, with the handling forces being acceptable in both opening and closing.
Gas Spring and gas strut installation handling forces chart. Crossover at 20 degrees Celsius. Angle of opening (Degrees) vs. force (N).
Crossover at 20°C
Gas Spring and gas strut installation handling forces chart. Crossover at 65 degrees Celsius. Angle of opening (Degrees) vs. force (N).
Crossover at 65°C
However, when the ambient temperature increases to 65°C (for example, in the case of an engine cover application), lift will occur almost instantaneously. This behaviour is unpredictable to the operator, in some cases proving dangerous.
It is also evident that handling effort to close the application increases. For these reasons it is important that understanding of what the ambient and ‘normal’ operating temperatures of an application will be, so if necessary, a suitable compromise can be made in handling performance.
Should dampers be mounted rod up or rod down? The answer to this is dependent on whether the damper is a compression or extension damper; each having specific orientations and should be mounted as follows:
Extension damper and compression damper side by side comparison. Extension Damper (Left), Compression Damper (Right).
Extension dampers should be mounted ‘rod down’ to ensure consistent damping throughout the stroke, if these are mounted ‘rod up’, this will result in little or no damping.
On the contrary to extension dampers, compression dampers should be mounted ‘rod up’ to ensure damping is consistent throughout. If instead they are mounted ‘rod down’, this will again result in little to no damping. Lubrication of the main seal is not a problem due to the high volume of oil used in the damper.
Mounting: Practical Application
Camloc’s preference when positioning is to allow 10mm of unused stroke. The reason for this is to allow for the stack up of manufacturing tolerances in the application, preventing “bottoming out” before the lid is fully closed.
10mm of unused stroke example.
Preventing Instant Lift
Positioning the moving mounting point so that it creates an over-centre condition when the lid is closed will aid in reducing instant opening of the lid.
The figure below highlights two examples; one of a position which will lead to instant lift (on the left) and one which is over-centre and will reduce the potential of instant lift (on the right).
Preventing instant lift spring position example.
Reducing Handling Forces
To help reduce handling forces, the following strategies can be used:
Move the mounting point Y1 closer towards the pivot by reducing the stroke.
Consider whether the opening angle could be reduced.
Move fixed mounting point Y2 to a more suitable location.
Reducing handling force example.
For more technical information on mounting and other commonly asked queries, head over to our whitepapers section. For information on our product range, please visit our Products Page.