Q: What is pre-load in relation to electric locking devices? Can doors really jam through a combination of pressures and surface friction? Is pre-load a serious risk and what steps can be undertaken to ensure it does not occur?
A: A loose definition of pre-load is the combined friction of environment and components on an entry door. Pre-load is caused by multiple factors relating to a door, a door frame and its environment, which can create friction or sticking in the operation of an electric strike. Often the issues are caused by deformed doors, falling doors, loosening of lock componments like the strike plate over time, the addition of door seals that change mechanical characteristics, operation of air-conditioning and other factors that create pressure on a door.
Air pressure, or stack pressure, is a complex science and is governed by wind, HVAC systems and local thermal air currents. Stack pressure can test the best engineers, especially in high rise applications, where it can make doors very hard for people to open and impossible for automatic closers to shut. If this is the issue facing your installation team, you need to ensure management understands the issue has nothing to do with access control hardware.
It’s fair to observe that if the locking components have been correctly installed, pre-load often represents a collection of issues that don’t relate to access control hardware at all. Regardless, pre-load does exist in electric locking and in mechanical locking applications, too. When you add pre-load to door weight, air pressure and the resistance of door closers, the force required to open a door may be greater than a child or an elderly person could manage.
Overcoming pre-load is usually possible by shaking the door but this isn’t an acceptable fix in a professional application. Installers need to ensure their locking devices – strikes and plates – are installed perfectly. If door or frame needs trimming, this should be undertaken. Many strikes are sold with pre-load ratings that guarantee they will operate reliably over millions of cycles despite preloads of 25kg or more. While such reliability is worth having, overcoming causes of pre-load is just as important and in most applications, this should be possible for professional installers and locksmiths to achieve.
If pre-load is still a problem, it should be taken away by installing a mechanical lock with lever handles in addition to the strike to remove pre-load pressure from the electric strike. Alternatives include automatic sliding doors and automatic opener/closers. There may be some applications that benefit from the use of magnetic locking devices. If pre-load cannot be removed, pre-load-rated strikes guarantee operation even when a door is under constant heavy load.
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