Controlled retensioning of the chains has a very positive effect on chain life. Extreme retensioning should be avoided just as much as excessive slack. A guideline figure would be approx. 5 % of the actual chain pulling force. In the case of chains running in parallel both strands must be tensioned equally, at best by a common shaft for both sprockets. If no automatic chain tensioning device is available the chain must be adjusted by hand, by altering the distance between the sprocket shafts. A further possibility with long drives is to shorten the chain by removing individual links, provided that wear is relatively light. Various tools are available for dismantling and reassembling the chain. These tools are available for parallel pin and shouldered pin chains. Before relubrication the chains and sprockets should be cleaned to remove obstinate contamination and to permit the lubricant to penetrate via the back of the plates. The surface of the chain can also be cleaned using an apropriate solvent. Complete immersion and washing is not recommended as the cleaning agent does not evaporate completely from within the chain and thus the penetration of the fresh lubricant is obstructed.