5.4　Jack

5.4.1　Jack shall be maintained on a regular basis，and shall be inspected prior to use.

5.4.2　Any screw jack or rack jack shall be removed from service if the thread of its bolt and nut or rack has an abrasion exceeding 20%.

5.4.3　Oil used for hydraulic jack shall be clean，and shall not be less than the rated quantity.If hydraulic jack is operated at an ambient temperature of above-5℃，10#engine oil shall be used，and if hydraulic jack is operated at an ambient temperature of-5℃or below，spindle oil or transformer oil shall be used.

5.4.4　Jack shall have solid support，and shall have its top working area determined by verifying the local stability of the workpiece.

5.4.5　When a jack is in service，the height of safety block shall be adjusted while the workpiece is raised or lowered.

5.4.6　Where multiple jacks are used to lift one workpiece，jacks of consistent specification and type shall be selected to ensure coincident action and smooth raising/lowering，and the load for each jack shall not exceed 80% of its rated capacity.

5.4.7　Jack shall be operated to enable the acting force to be transmitted through its load-bearing center.