Mechanical Motion Control Components and Subsystems Understanding How Components Effect System Performance
Mechanical Motion Control Components and Subsystems Overview: Bearings Linear Bearing Technologies Introduction to Drive Assembly Components for Precision Motion Control Mechanical Advantage and Motion Control Repeatability vs Accuracy
Mechanical Motion Control Components Guidance Drive Train Motor EM Actuator
Mechanical Motion Control Components Carriage (Moving Load) Drivetrain Base (Fixed) Bearings/Guidance
Mechanical Motion Control Components Piston/Guidance Rod (Moving Load) Rod Bearing Base (Fixed)
Mechanical Motion Control Components Bearings/Guidance Carriage (Moving Load) Drivetrain Base (Fixed)
Bearings Bearing 3a: an object, surface, or point that supports b: a machine part in which another part turns or slides -Merriam-Webster Dictionary Simple bearings generally are used to allow a single degree of freedom in motion while limiting all others
Bearings Bearings Sliding Element Rolling Element Fluid Element
Bearings Sliding Element Bearings: Also known as plain bearings Simplest bearing Uses sliding surfaces Low cost Low life (relative to other bearing technologies) Highly customizable Many material options
Bearings Rolling Element Bearings: Typical construction Inner race Outer race Rolling elements Cage Seals Low friction High load capability Long and predictable life Widely used in motion applications Uses rolling and fixed components Many configurations available
Bearings Fluid Element Bearings: Also known as hydrostatic and hydrodynamic bearings Hydrostatic bearing fluid or gas is pressurized, typically with a pump, to support the load Hydrodynamic bearing fluid present in the bearing interface is pressurized by high speed of the shaft journal and supports the load High load High speed High precision High cost
Bearings Bearing Overview Chart: Bearing Type Friction Load Capability Speed Precision Life Span Relative Cost Sliding Element Moderate - Variable based on Materials Low to Moderate - Impacted by Speed Low to Moderate - Impacted by Load Low to Moderate Low - Variable based on Materials Low Rolling Element Low Moderate to High - Variable Styles High to Very High - Variable Styles Moderate to High Moderate to High Low to Moderate Fluid Element Low to Very Low Very High Very High Very High Very High High
Mechanical Motion Control Components and Subsystems Overview: Bearings Linear Bearing Technologies Round Rail Profile Rail Roller Bearing Others: Cross Roller, Ball slide Fluid Bearings (Pneumatic and Hydraulic)
Linear Bearing Technologies Bearing 3a: an object, surface, or point that supports b: a machine part in which another part turns or slides -Merriam-Webster Dictionary Simple bearings generally are used to allow a single degree of freedom in motion while limiting all others
Linear Bearing Technologies Round Rail Linear Bearings: Allow for two degrees of freedom Simplest of linear bearing options Highly customizable Low stiffness Low Cost System drag based on application Typically a lower life option
Linear Bearing Technologies Profile Rail Linear Bearings: Allow for one degree of freedom May include wear compensation Low to moderate stiffness Can be highly customizable System drag based on application Low to moderate cost (varies by configuration)
Linear Bearing Technologies Roller Bearing Linear Bearings: Allow for one degree of freedom High stiffness High precision Limited ability to customize Low drag/friction Moderate to high cost Not typically for dirty environments
Linear Bearing Technologies Ball Slide Linear Bearings: Allow for one degree of freedom High stiffness High precision Limited ability to customize Low drag/friction Moderate to high cost Not typically for dirty environments
Linear Bearing Technologies Fluid Linear Bearings: Pneumatic and Hydraulic Allow for one degree of freedom Can Have High stiffness High precision Limited ability to customize Ultra Low drag/friction High cost Not used in dirty environments
Linear Bearing Overview Table Technology Low Friction Wear Smooth Motion Normal Loading Moment Loading Long Stroke Audible Noise Harsh Environments Low Cost Sliding Rolling Element Guide Rod with Plain Bearings Spline Shaft with Plain Bearings Rails With Plain Bearings Roller Bearings Ball Slides Fluid Pneumatic Hydraulic Good = Better = Best =
Mechanical Motion Control Components and Subsystems Overview: Bearings Linear Bearing Technologies Introduction to Drive Assembly Components for Precision Motion Control Screws Belt Drives Gear Reducers Couplings
Drive Assembly Components Lead Screws: Sliding element load propulsion High resolution of linear motion Moderate to high stiffness High repeatability Backlash on plain assemblies Anti-backlash and wear compensation available as options Can be speed limited (screw whip) Low to moderate cost
Drive Assembly Components Ball Screws: Similar to lead screws, but have rolling element load propulsion Higher efficiency then lead screws Higher accuracy then lead screws (control scheme dependent) Higher stiffness Higher load capacity Preload options available Can be speed limited (screw whip) Moderate to high cost
Drive Assembly Components Belt Drives: Rotating pulley drives a belt attached to a guide for linear motion High speed High acceleration Low noise Low to moderate precision Low cost Many configurations available
Drive Assembly Components Gear Reducers: Used to manipulate torque and angular velocity Common Options Spur Planetary Worm
Drive Assembly Components Spur Gearbox: Pairs of straight cut gears Relatively low torque capacity Relatively high backlash Moderate efficiency Moderate speed May have an offset output shaft Low cost, simple design
Drive Assembly Components Planetary Gearbox: Multiple planetary gears revolve around a sun gear (higher complexity) Planetary gears turn within the planetary gear carrier (outer ring gear) Low backlash High efficiency (as low as 3% losses per stage High load capability (multiple gears share the load) High speed Higher cost
Drive Assembly Components Worm Gearbox: Worm and worm gear sets Low efficiency High torque capacity Low speed Large gear reductions in a single stage Not typically backdrivable Offset output shaft Low cost, simple design Worm Gear Worm
Drive Assembly Components Couplings: Used to transmit power in many rotary to rotary and rotary to linear motion configurations Rigid Spider Oldham Helical
Drive Assembly Components Couplings comparison table: Torsional Rigidity Axial Misalignment Angular Misalignment Radial Misalignment Long Life Low Reactionary Forces Rigid Spider Oldham Helical Low Cost Fair = Good = Better = Best =
Mechanical Motion Control Components and Subsystems Overview: Bearings Linear Bearing Technologies Introduction to Drive Assembly Components for Precision Motion Round Rail Mechanical Advantage and Motion Control Screws Belt Drives Gear Reducers
Mechanical Advantage and Motion Control Governing Equations of Screws T 2 π e L Force F = T = Example: Find required motor torque for the following: or Where: F = Linear Force/Thrust L = Lead (advancement per revolution) T = Torque e = efficiency L F 2 π e F = 200N Lead = 2 mm/rev e = 35% T = 2mm 200 N rev 2 π 0.35 1 m = 0.182Nm 1000 mm
Mechanical Advantage and Motion Control Governing Equations of Screws Force V = ω L or ω = V L Speed Where: V = Linear Velocity L = Lead (advancement per revolution) ω = Angular Velocity Example: Find required motor RPM for the following: V = 3.5 in/sec (target speed) Lead = 0.25 in/rev ω = 3.5 in sec 0.25 in rev 60 sec 1 min = 840RPM
Mechanical Advantage and Motion Control Governing Equations of Screws Force Speed Critical Speed CS = MF 4.7 106 D r + D 0 2 L 2 Where: CS = Critical Speed MF = Screw Mounting Factor Dr = Screw Root Diameter Do = Screw Outside Diameter L = Length between screw supports Mounting Factors: 0.36 Rigid/Free 1.00 Simple/Simple 1.47 Rigid/Simple 2.23 Rigid/Rigid *Assumptions Length units are in inches and the material has a modulus of elasticity of 29 Mpsi Simplified from the Rayleigh-Ritz method, apply a 0.75 multiplier or lower for factor of safety
Mechanical Advantage and Motion Control Governing Equations of Screws Force Speed Critical Speed Example: Find the maximum allowable speed of a screw with a 0.125 in/rev lead, 0.375in OD, 0.270in RD, a length between supports of 14in and a simple/simple mounting factor. CS = 1.00 4.7 10 6.375 +.270 2 14 2 = 7,733RPM 7,733RPM.75 = 5800RPM Linear Speed = 5800rev min.125 in 1min rev 60sec = 12 in sec *Assumptions Length units are in inches and the material has a modulus of elasticity of 29 Mpsi Simplified from the Rayleigh-Ritz method, apply a 0.75 multiplier or lower for factor of safety
Mechanical Advantage and Motion Control Governing Equations of Belt Drives Force F = T e R p or T = F R P e Example: Find required motor torque for the following: Where: F = Linear Force/Thrust Rp = Drive Pulley Radius T = Torque e = efficiency F = 200N Rp = 25mm e = 90% T = 200N 25mm 0.9 1 m 1000 mm = 5.5Nm
Mechanical Advantage and Motion Control Governing Equations of Belt Drives Force Speed V = ω π P D Where: V = Linear Velocity PD = Pulley Diameter ω = Angular Velocity or ω = V P D π Example: Find required motor RPM for the following: V = 3.5 in/sec (target speed) PD = 2 in ω = 3.5 in sec 2 in π 60 sec 1 min = 33.4RPM
Mechanical Advantage and Motion Control Governing Equations of Belt Drives Force Speed Pulley Ratios V 1 V 2 = D 2 D 1 and T 1 T 2 = D 1 D 2 Where: V1 = Velocity of Pulley 1 V2 = Velocity of Pulley 2 T1 = Torque of Pulley 1 T2 = Torque of Pulley 2 D1 = Diameter of Pulley 1 D2= Diameter of Pulley 2
Mechanical Advantage and Motion Control Example: If the red pulley has a radius of 1in, the blue pulley has a radius of 2in answer the following: 1) What is the blue pulley RPM if the red pulley spins at 10RPM? 2) What is the torque applied to the shaft of the blue pulley if the red pulley is driven with 15inlbs (assume 100% efficiency) V Blue = 10 RPM 1in 2in = 5 RPM T Blue = 15 in lbs 2in 1in = 30 in lbs
Mechanical Advantage and Motion Control Governing Equations of Gear Reducers Torque GR in GR out = T out T in Tout = T in GR in e GR out Where: GRin= Input Scalar Value of Gear Ratio GRout = Output Scalar Value of Gear Ratio T in= Input Torque Tout = Output Torque e = efficiency Example: Find required motor torque for the following: Tout = 200Nm Ratio = 25:1 e = 90% T in = 200 Nm 1 25 0.9 = 8.89Nm
Mechanical Advantage and Motion Control Governing Equations of Gear Reducers Torque Speed GR in GR out = V in V out Vin = V Out GR in GR out Where: GRin= Input Scalar Value of Gear Ratio GRout = Output Scalar Value of Gear Ratio Vin= Input Velocity Vout = Output Velocity Example: Find required motor speed for the following: Vout = 75RPM Ratio = 50:1 e = 85% V in = 75 RPM 50 1 = 3750 RPM
Accuracy and Repeatability Accuracy and Repeatability Both are related to the level of precision in a system The terms are not interchangeable, each is unique Repeatability: The ability to reproduce the same result given the same input conditions Accuracy: The variation of actual achieved results compared to the target or goal. If a process is repeatable it can likely be adjusted to be accurate
Mechanical Motion Control Components and Subsystems Review: Bearings Linear Bearings Drive Assembly Components for Precision Motion Accuracy and Repeatability Mechanical Advantage and Motion Control Equations T = FL 2πe F = T e R p CS = MF 4.7 106 D r + D 0 2 L 2 T in GR in e GR out
Mechanical Motion Control Components Carriage (Moving Load) Drivetrain Base (Fixed) Bearings/Guidance
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