MACHINE BUILDING, FRAMEWORKS & SAFETY FEATURE In many safety-critical applications, safety brakes are
often the only means of safeguarding against crashes or the inadvertent descent of suspended loads. Here, Mayr power transmission looks into the points to consider when specifying safety brakes for machines and systems
SAFETY BRAKES: MAKING THE RIGHT SELECTION TO PREVENT INJURY OR DAMAGE
S
uspended loads in machines and systems – including machine tools, handling systems,
assembly facilities, lifting devices, cranes and stage winches – represent a substantial risk, especially when people have to stand beneath them. Incorrect selection or dimensioning of safety brakes can therefore lead to serious injury or damage to property. So, during the design phase, it is important to exclude the risk of both an inadvertent descent of loads and long stopping distances. For safety-critical applications,
safety brakes designed in accordance with the fail-safe principle are the ideal choice, as these generate the braking force through thrust springs and are closed in de-energised condition. Reliable motor drives, whilst also being in a position to decelerate vertically- moved loads to a standstill in case of power supply failure (SS1 operation), cannot subsequently hold loads suspended without a supply of power. Another criterion is that the basic safety principles
BRAKING TORQUES For high levels of reliability, the static and dynamic braking torques also need to correspond to the customer’s specifications. With it being necessary to check these application
“Incorrect selection or
dimensioning of
safety brakes can lead to serious injury or damage to property”
conditions in trials, it is highly recommended that users obtain information about the manufacturer’s test field, and question whether the brakes are subjected to tests that are as realistic as possible: Does the manufacturer also subject the safety brakes to climatic tests with realistic temperature and air humidity ranges? Are parameters such as braking torque, speed, temperature, coefficient of friction and friction work determined and checked under various conditions? Are the friction value progression and the wear characteristics of the friction system evaluated?
Only in this way is it possible to verify that the safety brake has been
are applied, including the reliable spring force, with manufacturers relying on springs which comply with EN ISO 13849-2. In addition to sufficient spring guidance, it is important that the coil distance on the thrust springs is smaller than their wire diameter. Finding out about the safety factor under continuous load and the dimensioning in the dynamic fatigue strength range are also important.
THINKING ABOUT STOPPING DISTANCES Another factor to consider is the stopping distance, with short stopping distances important for safety- critical and vertical axes in particular. The switching times of the brake are decisive for the braking distance because during the ‘free-fall time’, until the brake closes and deceleration takes effect, the mass also accelerates – possibly to such extremes that the permissible values of the brake are exceeded. However, alongside short stopping times it is also
necessary to check that the switching times can be maintained throughout the service life of the brake. To achieve the greatest possible level of safety it is also important that the brakes are equipped with a reliable switching condition monitoring system.
correctly selected and dimensioned for the application concerned. At renowned brake manufacturers, the data
gained from the trials flows into a calculation tool for safety brakes, meaning they can make reliable statements on how the braking system will behave under certain conditions, and what is and isn’t possible in a special application, for example. These manufacturers will also inspect every safety brake which leaves the plant.
PRODUCTS FOR DIFFERENT APPLICATIONS With the application fields for safety brakes being so diverse, a wide selection of rotatory and linear brakes are available. These include brake designs for rotating shafts,
redundant double brakes, disk brakes, brake solutions for axes with rack-and-pinion drive, linear brakes for profiled rail guides, linear brakes for piston rods in pneumatic, hydraulic and electromagnetic design, and more. Some manufacturers are also able to design the brakes to meet application demands.
Mayr power transmission
www.mayr.com
ROBA-stop-silenzio dual circuit brakes are said to be the quietest safety brakes on the market
DESIGN SOLUTIONS | JUNE 2019 15
The ROBA-topstop safety brake is a ‘tried and tested component’ in terms of category 1 acc. DIN EN ISO 13849-1
The backlash-free safety brake ROBA-guidestop acts directly on the moving mass of the vertical axes
As a compact brake unit, the ROBA-linearstop can be integrated easily and quickly into existing machines and system designs
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