Question about Bernina Sewing Machines
How do I check the belt on a Bernina 708
Setting the belt tension on the 707-708-717 machines with the old rubber "Continental" belts:
1. Make sure that the hand-wheel spins freely on the main-shaft bushing stub when the release knob is loosened.
2. Assuming that you are using the original 441mm "Continental" rubber v-belts, the setting is made by means of loosening a 5mm Allen socket cap screw at the base of the step pulley support.
NOTE: The hand-wheel, stepped pulley, and motor pulley v-grooves must be clean. This is to say, that they cannot have any particulate or threads wrapped around the grooves.
4. The pulley will be automatically pre-tensioned to a 7 o'clock position by means of a spring to maintain the stepped pulley position. By pulling the belt toward this 7 o'clock position, the pulley tension can be adjusted and then tightened by means of the fixing screw.
5. It is important to know that both the upper and lower belt tensions can be independently adjusted so as to allow the hand-wheel to spin freely for bobbin winding without having the hand-wheel drive the machine. This is done by adjusting the step pulley support + or - radially, near the 7 o'clock position.
If the hand-wheel drives the machine with the upper belt loosened, then either the belt is too tight or the hand-wheel bushing and main-shaft stub need cleaning and lubrication.
Note: Lubrication was once the standard factory procedure with a class 68 oil, but experience dictates a dry lubrication with graphite. Silicone lubes are not recommended.
6. Do not over-tighten the 5mm Allen fixing screw as the 5mm threaded frame boss can be broken off.
Adjusting with the original Polyurethane v-belts on models 807-801-810-817-811 machines:
1. Follow the same procedure as mentioned but allow the step pulley's positioning spring provide the belt tension. Nothing more is required here.
NOTE: Sometimes these belts and pulleys get oily thus allowing much slippage. A simple cleaning of the belts and pulleys with mineral spirits can solve this problem.
NOTE: One can find after market belts that serves to drive these machines but not without some problems as they are usually wider belts and don't fit the pulleys well. In this case some adjustment will be necessary.
Posted on Jun 24, 2014
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Posted on Jan 02, 2017
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