Frequently asked questions

SKF Vibracon

What is an SKF Vibracon chock?

The SKF Vibracon chock is a universal adjustable steel chock. Universal refers to the fact that a SKF Vibracon chock can be used instead of rigid steel chocks, shims or epoxy resin chocks.
Adjustable refers to the possibility of aligning and re-adjusting the chock at every re-alignment event. The name of the chock ”Vibracon” suggests that the elements can absorb vibrations like a vibration damper. We want to point out that the SKF Vibracon chock is absolutely not a vibration damper (resilient mount).

General

How long have SKF Vibracon chocks been in use?
The SKF Vibracon chocks were designed and patented by the company Machine Support, which was acquired by SKF in the year 2000. Ever since the SKF Vibracon chocks have gained in popularity in a wide variety of industries.
How is the vibracon supported for marine applications?
The SKF Vibracon Original and SKF  Vibracon Low Profile chocks are approved by all major shipbuilding classification societies and used by a large number of original equipment manufacturers.
In addition, SKF is able to prepare the mounting proposals which can immediately be used to obtain a plan approval from the class society of choice.
Is the SKF Vibracon chock approved by classification societies?

Yes, the chock is approved by all major shipbuilding classification societies. However, it is always necessary to submit a calculation and a sketch of the chocking arrangement for approval for each application. This is mainly necessary for propulsion units and for some societies, also for auxiliary equipment. Contact your local dealer or SKF Maintenance Products for assistance.

The SKF Vibracon is type approved by the following class societies:

  • ABS
  • Bureau Veritas
  • CCS
  • DNV - GL
  • Lloyds Register of Shipping
  • Rina
Are there other relevant certificates that are of use?
  • Material Certificates
    All our Vibracon products are supported with Material Certificates. When the batch number engraved on the bottom ring is submitted to vibracon@skf.com, you will receive the material certificates by return email.
  • ISO 9001 Quality Management System
    In case a copy of the ISO 9001 certificate is required please email vibracon@skf.com.
  • Certificate of Origin or Certificate of Conformity
    In case a “certificate of origin” or “ Certificate of Conformity” is required you can sent an email with the necessary information (name exporter, delivery details, reference numbers, products, size & weight) to your local SKF contact person or to vibracon@skf.com.
  • Asbestos Free Declaration
    In case a “Asbestos Free Declaration” is required, please send the concerned document to vibracon@skf.com.
  • Dual-use
    Standard SKF Vibracons are not concerned by CDR 1382/2014 / 428/2009 and therefore not classified as “Dual Use”.
What are the benefits of SKF Vibracon chocks?

In packaged units (like generator, compressor, turbine and pump sets) the benefits are:

  • No parallel or angular soft foot problems, even when the skid is twisted;
  • No machining of the base frame;
  • No on site problems to arrange or install shims;
  • Re-adjustable; 
  • In combination with laser alignment equipment, alignment time can be reduced by 50%;
  • Existing chocking systems, steel chocks or epoxy resin with a height of only 20 mm, can be replaced by the SKF Vibracon Low Profile configuration (depending on SKF Vibracon size).

Onboard ships for chocking propulsion units and auxiliary equipment:

  • No curing time like epoxy resin chocks;
  • No time consuming fitting of rigid steel chocks;
  • Chocking can be carried out when the ship is still in the dry dock, the alignment check and (if required) realignment has to be carried out after the ship has been launched;
  • No parallel or angular soft foot problems, even if the foundation is not machined;
  • No machining of foundation, only locally (where the elements will be fitted) machining if required.
What is more cost effective: SKF Vibracon, Steel chocks, Epoxy Resin or Shims?

In most of the applications the SKF Vibracon chocks will be more economic because of:

  • Reduced installation and chocking time
  • Always a perfect fitted chock
  • No soft foot problems
  • Installation of the SKF Vibracon chocks doesn’t require special skills or tools
  • Usually no machining of foundations or base frames is required
Where are the SKF Vibracon chocks applied?
SKF Vibracons are being used in a wide variety of applications and industries. The common denominator is that most applications are rotating machinery where alignment is critical and downtime is costly.
In production plants the elements can be used below all kinds of rotating equipment, and also in production lines which need accurate alignment.
When SKF Vibracon chocks will be mounted on a concrete foundation, the maximum allowable surface pressure of the existing concrete has to be checked. In case the surface pressure will be too high, it is possible to reduce the surface pressure, with the use of an additional steel plate of sufficient surface and thickness, to an acceptable level. The SKF engineers will be pleased to make a recommendation.
Do we need special skills or tools to use the Vibracon chocks?
Special skills or tools are not required to use SKF Vibracon chocks. SKF Maintenance Products always advises you to contact your local dealer for support during the first time installation of the elements. Also, please request an SKF Vibracon installation manual.
The use of laser alignment equipment is recommended but not necessary.
How do I order Vibracon chocks?
SKF Vibracon chocks are kept on stock at SKF. However, it is recommended to check the delivery time before ordering. The local authorized distributors may also have SKF Vibracon products on stock.
How to handle non-standard product orders/enquiries?
Engineering enquiries with a scope outside our standard range can be sent to vibracon@skf.com. The SKF Vibracon engineers will revert with a proposal and product recommendation. In addition, they will make sure the correct product will be made available.
Myself or my contact person at SKF cannot find the right product in the SKF systems.
Please note that the syntax of the SKF Vibracon designations is critical and if miswritten will not return the correct results. At vibracon@skf.com, we are pleased to support.
What is the delivery time?
The majority of the SKF Vibracon products are kept on stock in sufficient quantities enabling turnaround delivery times. Please contact your local SKF contact person for support on lead times and stock levels.

Technical questions

What is the difference between machine load, element load, proof load and failure load?
Machine load
The machine load is the sum of the weight of the component + the dynamic forces operating on the SKF Vibracon. The total weight of the component to be divided by the number of SKF Vibracon Chocks under the component and the dynamic forces are calculated for the heaviest loaded position. These forces multiplied with a safety factor gives us the "Machine Load" which is the base for the Vibracon chock size selection.

Maximum element load
The maximum element load allowed (at the maximum height) on the SKF Vibracon chock during normal operation of the equipment, including the weight of the machine, operational loads and bolt force. The maximum element load includes a safety factor in order to accommodate additional forces during malfunction operation (Maximum element load > Machine Load + Bolt load).

Proof load
Proof load is the tested load which can be applied on the SKF Vibracon chock at the maximum height before plastic deformation will occur. Exceeding the proof load will permanently deform the chock, rendering it no longer adjustable.
Is it possible to chock a heavy component on an SKF Vibracon chock?

Yes, the primary forces for this Vibracon sizing consideration are:

Fweight    due to gravity
Ftorque due to the fact that the machine wants to rotate
Fbolt due to the tightening torque of the foundation bolt

Fbolt is approximately 6 till 10 times Fweight and even 10 till 15 times Ftorque.

This means that Fbolt is the critical force on the normal element. If we consider, for example, the SKF Vibracon SM 30 chock, we can see that we are tightening a M 30 bolt against a M95 thread (internal thread size of an SKF Vibracon SM 30). It is easy to understand that the SKF Vibracon chock can’t be damaged by the machine forces unless the element sizes are wrongly calculated or the foundation bolts are insufficiently tightened.
Can we align a machine with an SKF Vibracon chock?
The SKF Vibracon chocks are primarily meant for chocking. For aligning the machine, hydraulic tools or jack bolts are recommended. However, due to the configuration of the SKF Vibracon chocks they can be used for vertical adjustment under load under the condition that the right tools are being used. SKF will be pleased to advice on the possibilities and limitations through vibracon@skf.com.
Do I need to tack weld the SKF Vibracon chock to the top plate when installing?
There is no need to tack weld any part of the SKF Vibracon chock. In fact, we recommend to avoid welding in the direct circumference of rotating equipment. If welding is required take care for proper earthing!
How do we prevent the chock from rotating?
The SKF Vibracon chock will lock itself due to friction in the thread after the foundation bolts are tightened. It is not necessary to lock the chock in any other way.
Are there any limits to the chock height?
Yes, there are limits to the standard chocks. Please check in the table on the SKF Vibracon datasheet the minimum and maximum height of each type of chock. When designing a chocking arrangement, we recommend to consider the nominal height of the chock.
What to do if we want to have chocks with extended height?
In this case, it is possible to use an extended Vibracon model or an additional bottom ring. The maximum height of the chock should not exceed the diameter of the chock.
When to use an additional bottom ring and when an extended Vibracon model?
There is no specific preference for use of one or the other. The only and major difference is that an additional bottom ring introduces another component in the chock whereas the extended model consist of 3 components only.
When to use an extended top plate?
The extended top plates are a recommended product for application where Vibracon Low Profiles are used and extra chock height is needed or in cases of geometric constraints.
What to do if we want to have chocks which are lower than the minimum height?
It is allowed to reduce the SKF Vibracon Original to a minimum reduced height which is mentioned in the SKF Vibracon datasheet.
Reducing the height can be done on a lathe by machining the Centre and Bottom part of the chock. Keep in mind that the adjustment capabilities also will be reduced. At the minimum reduced height, only 1 mm adjustment capability has been left.
Can the SKF Vibracon chock be installed upside down?
Yes, this is possible and is a perfect solution if the design of the foot or foundation so dictates.
Is it possible to enlarge the bolt hole of the SKF Vibracon chock?
To do so there is a need for approval from SKF and -for marine applications- the classification society. Based on the specific design, the SKF Vibracon engineering department will provide recommendations.
If we want to use fitting bolts, how should we do this?
In general, it is not necessary to ream through the SKF Vibracon chock. Reaming only the foot of the machine and the foundation usually will be sufficient. In some situations, (for example in cases where no side and collision stoppers are installed) we also advise to ream the SKF Vibracon chocks. In case of the latter it has the recommendation to install the Vibracons upside down in order to prevent the Vibracon to rotate down during reaming.
If we want to use dowel pins, how should we do this?
After the alignment and the securing of the foundation bolts, weld down a block of steel (which is just 1 mm lower than the chocks) near at least one of SKF Vibracon chocks or between two chocks. Drill a hole through the foot of the machine into this block. Now the dowel pin can be installed.
What information is needed to make a calculation and a sketch of the chocking arrangement?
To obtain a, free of charge, SKF Vibracon chocking proposal it is necessary to fill in a checklist, which can be found on www.skf.com.
What bolt torque should we apply?

SKF has developed bolt torque calculation software in close cooperation with original equipment manufacturers and classification societies. The philosophy of the calculating software is to reach a minimum bolt elongation (stretch) of 0.25 mm in the bolted connection. The bolts are to be tightened within the elastic range of the bolt material, in general SKF advices a maximum bolt tension related to 75% of the yield strength of the bolt material.

The values stated in our SKF Vibracon brochures are based on the above mentioned calculations rules. The mentioned values are valid for bolts with coarse thread, material grade 8.8, yield strength > 640 N/mm², oil lubricated thread coarses and nut mating surfaces without slide additives. As a service to our customers, SKF offers you a chocking proposal, with the advised tightening torque included, for your application free of charge! This and further support can be obtained through vibracon@skf.com.

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