Tech Tip: PT Accessories

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When designing a new drive/conveyor system, when is it necessary to use overload protection?

Not all drives and conveyor systems will require overload protection based on the specific application. Overload protection is an important addition to any drive/conveyor system design when there are concerns for any of the following:
•   Opportunity for human error – When a system is being operated by a human and not by an automatic control system,       there is more chances for human error to affect operations. Overload protection can provide necessary protection to
     prevent human error from preventing equipment failure.
•   Opportunity for material jamming – When an application has the opportunity for material jams, an overload protector
     is very necessary to prevent damage to equipment caused by these jams.
•   Unreliable electrical supply – In areas or old buildings with unreliable electrical supply, phase loss, electrical surges,
    short circuits, and/or ground faults can cause damage to motors without proper electrical overload protection.
•   Critical componentry that is less robust than the rest of the system – If an application has critical components that are
     less robust than the rest of the system, these critical components will be the first to fail in the case of overload. Proper       mechanical overload protection can protect these critical components from premature failure caused by overload       
     conditions. 

How do I determine whether to use electronic or mechanical overload protection?

•  economically priced
•  solid state electronics for long lifetime
•  easy to install
•  maintenance free
•  set-it and forget-it
•  no equipment retrofit required
•  can be panel mounted and used in any atmosphere when in a lifting/hanging application and power is cut,           motor brake can hold load.

•  motor size limitations
•  can struggle to detect impact loading
•  when used with reduction ratios above 80:1 damage can occur in reducer prior to motor experiencing                   increased torque.

- Mechanical Overload Protectors

•  unlimited made to order size range
•  instant torque protection
•  no issues reacting to impact loads
•  easily adjustable
•  repairable
•  can be installed directly onto critical/weak component for ultimate protection

•  more expensive than electronic protectors
•  often require equipment retrofitting to fit unit
•  atmosphere limitations due to open construction
•  has wear components
•  when in a lifting/hanging application and unit disengages, there is no method to hold load in place.

How do I determine what kind of Mechanical Overload Protection to use?

Mechanical overload protection comes in two main methods:

• Friction Disk style mechanical overload protectors work using the relationship between perpendicular force and friction. Friction disk overload protectors have special friction disks that are pressed against a sprocket face using adjustable spring force. This spring force creates a frictional force between the disk and driven sprocket. When the torque experienced by the driven sprocket exceeds the frictional force between the disc and sprocket, the sprocket slips and no torque is transmitted. As soon as the torque is removed and the frictional forces exceed the driven member torque, the unit reengages and transmits torque again.
• Interference style mechanical overload protectors work by some form of interference fit with a spring force. One common method of interference style overload protection is using ball-detent, where a ball is pressed into a detent using a disc spring. When the torque experienced by the bolted on driven member (sprocket, gear, pulley, etc.) exceeds the spring force, the ball slips out of the detent and no torque is transmitted. When the excessive torque is removed and the balls come back into position with their respective detent, the unit reengages and transmits torque again.

How do I determine what kind of Electronic Protection Overload system to use?

Electronic Overload Protection comes in three main methods:

1. Shock Guards are standalone electronic devices that measure and monitor amperage draw of the motor. When the motor amperage exceeds a set value for longer than a set shock period, the shock guard sends a signal to the motor controller to cut motor power. Shock guards are able to react much more quickly than inverter drives, as quickly as 0.05 seconds, which provides greater protection. The downside to shock guards is the reduced ability to detect impact loads.
2. Shock Monitors are standalone electronic devices that measure and monitor motor power. Motorpower is a measurement that takes into account amperage, voltage and time for a more accurate portrayal of the torque experienced by the motor. When the motor power draw exceeds a set value for longer than a set shock time, the shock monitors send a signal to the motor controller to cut power. Shock monitors offer the quickest and most comprehensive electronic overload protection solution.
3. Inverter Drives often have features that can monitor the amperage draw of the motor and when the amperage exceeds a set value, the drive cuts power. The main downside to inverter drive overload protection is reaction speed; inverters react slower than Shock Guards or Shock Monitors. 

What are the most common types of overload protection used to protect critical power transmission components, and what are their main differences?

The two most common types of overload protection are electronic and mechanical protectors are:

• Electronic Overload Protectors work by measuring and monitoring the electrical draw of the motor. When the draw from the motor exceeds a set point, the electronic overload protector sends a signal to the motor controller to cut power to the motor.

• Mechanical Overload Protectors are installed to a driven rotating shaft and has some rotating element attached to it (sprocket, gear, pulley, coupling, etc.). When the torque experienced by the rotating element exceeds a set amount, the overload protector physically disengages the rotating shaft from the rotating element to prevent torque transmission. 

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.