Frequently asked questions
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).
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.
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:
- Bureau Veritas
- DNV - GL
- Lloyds Register of Shipping
All our Vibracon products are supported with Material Certificates. When the batch number engraved on the bottom ring is submitted to firstname.lastname@example.org, 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 email@example.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 firstname.lastname@example.org.
Asbestos Free Declaration
In case a “Asbestos Free Declaration” is required, please send the concerned document to email@example.com.
Standard SKF Vibracons are not concerned by CDR 1382/2014 / 428/2009 and therefore not classified as “Dual Use”.
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;
- 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.
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
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.
The use of laser alignment equipment is recommended but not necessary.
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 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.
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.
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.
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 firstname.lastname@example.org.