Ball Screw Drive Actuator for Electromechanical Door Latch
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Solution Overview
Problem
Existing electrified latch pullback mechanisms in push bar exit devices face challenges due to high friction losses in acme lead screws, which require larger motors or finer thread pitches, leading to size constraints and slower actuation times, and the need for additional springs for fail-safe operation, complicating the balance of drive forces and often making it impossible to fit within enclosed mechanism cases.
Innovation Solution
The use of a ball screw drive system, which reduces friction losses by employing bearing balls to transfer power between the screw and nut, allowing for a smaller motor and lower power consumption, and a new ball drive design that eliminates the need for additional parts and simplifies machining, enabling the system to fit within smaller enclosures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If acme lead screw is used to translate rotational motor output to linear force, then linear actuation is achieved, but friction losses are very high requiring larger motors
Solution Approach 1:
The patent replaces the acme lead screw mechanical system with a ball screw drive system. The ball screw uses ball bearings to reduce friction between the screw and nut, substituting the high-friction sliding contact of the acme screw with low-friction rolling contact. This mechanical substitution directly reduces friction losses and allows for smaller motor sizing.
2Loss of energy
If finer thread pitch is used to reduce friction losses, then friction is reduced, but actuation time becomes slower
Solution Approach 1:
The patent changes the fundamental parameter of the screw drive system from acme thread profile to ball screw profile. This parameter change enables the use of optimal thread pitches that balance friction reduction with acceptable actuation speed, unlike finer acme pitches that inherently slow actuation while reducing friction.
3Reliability
If springs are added to return leadscrew to starting point for fail-safe operation, then fail-safe operation is achieved, but device complexity increases
Solution Approach 1:
The patent replaces the acme lead screw system that requires spring mechanisms for fail-safe operation with a ball screw drive system. The ball screw's inherent properties allow for simpler fail-safe implementation without requiring additional spring components, thus reducing device complexity while maintaining reliability.
4Force
If larger motor is used to compensate for friction losses, then sufficient linear force is generated, but the motor cannot fit within the enclosed mechanism case
Solution Approach 1:
The patent substitutes the acme lead screw with a ball screw drive system, which dramatically reduces friction losses. This substitution allows for the use of a smaller motor that can generate the required linear force with significantly lower power consumption, enabling the motor to fit within the enclosed mechanism case constraints.
5Power
If ball screw drive is used to reduce friction losses, then smaller motor is required, but back-driving friction characteristics are different
Solution Approach 1:
The patent addresses the different back-driving friction characteristics of ball screws by designing the system to utilize these characteristics beneficially. The ball screw's ability to be back-driven with controlled friction is leveraged to simplify fail-safe mechanisms and reduce the need for additional components, converting what could be a disadvantage into a system advantage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The ball screw drive system enables smaller motor usage, reduced power consumption, and simplified design, allowing for efficient operation within size constraints while maintaining effective actuation and fail-safe mechanisms, overcoming the limitations of acme lead screws.
Implementation Method 1
The ball screw drive system enables smaller motor usage, reduced power consumption, and simplified design, allowing for efficient operation within size constraints while maintaining effective actuation and fail-safe mechanisms, overcoming the limitations of acme lead screws.
Implementation Method 2
The use of a ball screw drive system, which reduces friction losses by employing bearing balls to transfer power between the screw and nut
Data Source
AI summary
An embodiment is an electromechanical door latch system including a latch device configured for motion between a latched position and an unlatched position. A manually actuated mechanical apparatus is coupled to the latch device and configured to move the latch device to the unlatched position in response to force applied by a user to a component of the mechanical apparatus. An electromechanical actuator system moves the latch device to an actuator unlatched position in response to an electrical signal from an access control device. The electromechanical actuator system includes a control module, a motor, and a ball screw drive including a ball screw or threaded nut, with one coupled to the motor shaft and the other configured for connection to a latch mechanism, and at least one recirculating ball circuit including ball bearings for transferring torque from the motor to the ball screw drive.