FAQ

Frequently Asked Questions


Should a Lift Support be mounted "shaft up" or "shaft down"?
Should a Damper be mounted "shaft up" or "shaft down"?
What types of end fittings can be used when mounting a Lift Support or Damper?
How does temperature affect the life and performance of a Lift Support or Damper?
What is the expected life of a Lift Support?
How can a designer ensure the longest life for a Lift Support in an application?



1. Should a Lift Support be mounted "shaft up" or "shaft down"?

"Shaft down" is the preferred orientation for mounting a Lift Support. An optimum design would permit the support to be oriented shaft down through its entire actuation. There are several reasons for this:

In order to achieve the damping or "cushion" at the end of the Lift Support's stroke, the piston assembly inside of the Lift Support must travel through oil at the end of the stroke. "Shaft down" orientation ensures that the oil is in the proper location for damping to occur.

"Shaft down" orientation ensures that the shaft and sealing components are lubricated with every stroke of the Lift Support. This reduces seal wear and helps to inhibit corrosion.

Depending on the application, "shaft down" orientation may reduce the rate of gas permeation through the seal, which increases the useable life of the Lift Support.

Although inadvisable, Lift Supports may be mounted "shaft up", but this should be discussed thoroughly with an AVM representative.

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2. Should a Damper be mounted "shaft up" or "shaft down"?

The type of damping designed into the unit determines the mounting orientation of a damper. Extension and Compression dampers require specific orientations. AVM's patented Non-cavitating damper designs can be mounted in any orientation. Extension and Compression dampers should be mounted as follows:

Extension dampers should be mounted "shaft down" to provide consistent damping for the full stroke. If the damper is mounted with the shaft pointing up, the unit may experience inconsistent damping or no damping at all.

Compression dampers should be mounted "shaft up" to provide consistent damping for the full stroke. If the damper is mounted with the shaft pointing down, the unit may experience inconsistent damping or no damping at all. Lubrication of the seal is not a problem due to the high volume of oil contained in a damper.


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3. What types of end fittings can be used when mounting a Lift Support or Damper?

Any type of connector that can be screwed or welded (Lift Supports only) to the shaft and/or body of the Lift Support or Damper should be suitable for use. When choosing a connector, the designer must take into account both tensile and compressive loading to ensure adequate strength.

The most common connector involves a ball and socket joint. While this joint style is available in many forms, they all allow rotation about the mounting point, which helps to prevent side loading on the support or damper. This is important because side loading can reduce the life of the product.

The simplest connector is a flat welded blade with a through hole. This connector has a low profile and can be very cost-effective, however, side loading can be a concern. This may be addressed by using an oversize hole, which allows for some rotation about the mounting point. Additional concerns with this connector include reduced wear and corrosion resistance.

The designer should also be concerned with the clearance around the mounting point. There should be enough room to easily mount and remove the unit from the application.


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4. How does temperature affect the life and performance of a Lift Support or Damper?

Temperature affects Lift Supports in two ways, output force change and increased susceptibility to gas loss.

As the temperature of the Lift Support changes, the internal pressure changes according to the relationship P1/T1 = P2/T2. Therefore, as the temperature increases, so does the internal pressure. As the internal pressure increases, so does the output force. For every 10C (18F) change in temperature, the output force changes 3.5 percent.

Very high or very low temperatures can adversely affect the Lift Support's ability to retain its gas charge. At very high temperatures, the permeability of rubber increases and the gas molecules may diffuse through the seal more quickly. Additionally, rubber compounds may begin to soften at elevated temperatures and lose their ability to seal properly. At very low temperatures, rubber compounds may stiffen and also lose their ability to seal properly. AVM's patented multi-lobe seal design and proprietary rubber compound help to minimize problems at temperature extremes. This allows AVM supports to perform reliably at temperatures ranging from 40C (-40F) to +80C (176F).

Within their specified range, Dampers are unaffected by temperature except for an increase or decrease in damping forces as the temperature rises and falls.


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5. What is the expected life of a Lift Support?

When estimating the life of a Lift Support, one must first determine how much force the support can lose before the application becomes unacceptable. The time it takes to lose this amount of force is considered to be the life of the support.

All Lift Supports lose output force over time. The rate at which force loss occurs varies greatly by application and manufacturer. Many factors affect the rate of force loss, such as: size of the support, orientation, amount of cycles, ambient temperature, vibration, and the geometry of the application. Considering all of the variables, it is very difficult to estimate life accurately without actual testing in the application.

As noted above, all Lift Supports will lose output force over time. It is recommended that the supports be periodically checked to ensure that they are functioning as intended. This inspection should be implemented as part of a planned maintenance activity, if possible.

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6. How can a designer ensure the longest life for a Lift Support in an application?

Orient the support "shaft down". As explained above, this will continually lubricate the seal and shaft and reduce permeation through the seal.

Utilize the largest gas volume possible in the support. In general, use the minimum stroke required with the largest body possible. In a support with a large gas volume, small gas losses are imperceptible in the output force. The temperature of the Lift Support should remain well within the temperature limit s. If temperature extremes will be encountered, it should be for a short duration and the support should not be cycled while at the extremes. Utilize the highest force Lift Support possible that still provides acceptable handle loads for the application. This will allow for some force loss without the loss of function of the application.

Provide multiple mounting locations so that the support can be moved to accommodate for force loss as the support ages.

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