generate questions: Steam engines frequently possess two independent mechanisms for ensuring that the pressure in the boiler does not go too high; one may be adjusted by the user, the second is typically designed as an ultimate fail-safe. Such safety valves traditionally used a simple lever to restrain a plug valve in the top of a boiler. One end of the lever carried a weight or spring that restrained the valve against steam pressure. Early valves could be adjusted by engine drivers, leading to many accidents when a driver fastened the valve down to allow greater steam pressure and more power from the engine. The more recent type of safety valve uses an adjustable spring-loaded valve, which is locked such that operators may not tamper with its adjustment unless a seal illegally is broken. This arrangement is considerably safer.[citation needed]
How many mechanisms does a typical steam engine have to keep boiler pressure from getting too high? {sep_token} What is restrained with a lever in the top of a boiler? {sep_token} What type of valve is used by recent safety valves? {sep_token} In an adjustable spring-loaded valve, what needs to be broken to allow an operator to tamper with it? {sep_token} Along with a desire for more steam pressure, what were early drivers looking to generate when they fastened safety valves down?