Power Jacks C-Series - Manual (Page 18)

18 www.powerjacks.com
Operation
5
5.2.15 Side Loads on a Screw Jack
Screw jacks are designed primarily to move and position loads and any side loads (loads not
acting axially on lead screw) should be avoided. The units will withstand some side loads,
depending on the diameter of the lifting screw and the extended length of the lifting screw.
Where side loads are present, the loads should be guided and the guides, rather than the screw
jacks, should take the side loads - particularly when long raises are involved. Even a small side
load can exert great force on the housings and bearings and increase the operating torque and
reduce the life expectancy.
5.2.16 Allowable Duty Cycle of Screw Jack
Because of the efficiency of conventional worm gear screw jacks, the duty cycle is intermittent
at rated load. At reduced loading, the duty cycle may be increased. The high performance
S-Series screw jacks have higher thermal efficiencies allowing generally 50% higher duty cycles
than conventional worm gear screw jacks. For detailed analysis consult Power Jacks Ltd.
5.2.17 Maximum Operating Temperatures For C-Series Screw Jack
Normal operation at ambient temperatures of up to 90ºC. Operations above 90ºC will require
special lubricants. For temperatures above 90ºC, the life of even special lubricants is limited.
Therefore consult Power Jacks on your application. For temperatures above 90ºC, advise Power
Jacks of full particulars of the duration of such temperatures. Power Jacks suggest that a
lubricant manufacturer be consulted for type of grease and lubrication schedule. As a general
rule, the screw jack unit should be shielded to keep ambient temperatures to 90ºC or less.
5.2.18 Minimum Temperature For C-Series Screw Jacks
With the standard lubricant and materials of construction, the screw jacks are suitable for use
at sustained temperatures of -20ºC. Below -20ºC, low temperature lubricant should be used
and no shock loads are present. Power Jacks application engineers must be consulted in these
instances for a recommendation. Screw Jacks with standard material of construction and
lubrication may be safely stored at temperatures as low as -55ºC.
5.2.19 Thermal / Heat Build-Up in a Screw Jack as it is operated
The duty cycle, the length of the screw, the magnitude of the load, and the efficiency of the
screw jack all have a direct influence on the amount of heat generated within the screw jack.
Long lifts can cause serious overheating. Note that high duty S-Series screw jacks have a higher
thermal capacity than conventional worm gear screw jacks.
5.2.20 Screw Jacks to Pivot a Load
A screw jack can be built to pivot a load by two methods:
5.2.20.1 Double Clevis Screw Jack
The screw jack can be furnished with a clevis at both ends (commonly referred to as a double
clevis screw jack). The bottom clevis is welded to the bottom end of an extra strong cover pipe,
which is fitted to the base of the screw jack. This cover pipe still performs its primary function of
encasing the lifting screw in its retracted portion.
5.2.20.2 Clevis - Trunnion Mounting
The screw jack is fitted with the pivot end fitting (e.g. Clevis) on the lead screw and a trunnion
mount adapter is bolted to the screw jacks base plate.
The design of the structure in which these types of screw jacks are to be used must be
constructed so that screw jack can pivot at both ends. Use only direct compression or tension
loads, thereby eliminating side load conditions