Experiments on Ultrasonic Lubrication Using a Piezoelectrically-assisted Tribometer and Optical Profilometer

Summary

We present a protocol for using a piezoelectrically-assisted tribometer and optical profilometer to investigate the dependence of ultrasonic wear and friction reduction on linear velocity, contact pressure, and surface properties.

Abstract

Friction and wear are detrimental to engineered systems. Ultrasonic lubrication is achieved when the interface between two sliding surfaces is vibrated at a frequency above the acoustic range (20 kHz). As a solid-state technology, ultrasonic lubrication can be used where conventional lubricants are unfeasible or undesirable. Further, ultrasonic lubrication allows for electrical modulation of the effective friction coefficient between two sliding surfaces. This property enables adaptive systems that modify their frictional state and associated dynamic response as the operating conditions change. Surface wear can also be reduced through ultrasonic lubrication. We developed a protocol to investigate the dependence of friction force reduction and wear reduction on the linear sliding velocity between ultrasonically lubricated surfaces. A pin-on-disc tribometer was built which differs from commercial units in that a piezoelectric stack is used to vibrate the pin at 22 kHz normal to the rotating disc surface. Friction and wear metrics including effective friction force, volume loss, and surface roughness are measured without and with ultrasonic vibrations at a constant pressure of 1 to 4 MPa and three different sliding velocities: 20.3, 40.6, and 87 mm/sec. An optical profilometer is utilized to characterize the wear surfaces. The effective friction force is reduced by 62% at 20.3 mm/sec. Consistently with existing theories for ultrasonic lubrication, the percent reduction in friction force diminishes with increasing speed, down to 29% friction force reduction at 87 mm/sec. Wear reduction remains essentially constant (49%) at the three speeds considered.

Introduction

Friction exists at the interface of two contacting surfaces when they slide or roll relative to each other. Friction usually occurs along with abrasive or adhesive wear.¹ Ultrasonics is the science behind high frequency phenomena, that is, waves traveling at frequencies above the acoustic range (20 kHz). The field of ultrasonics encompasses two fundamentally different regimes. One regime involves low intensity waves like those utilized in imaging processes such as medical ultrasound or non-destructive inspection of structures. The other is a high power regime in which high-energy waves are utilized to execute or assist engineering processes such as welding ....

Protocol

1. Development of the Modified Tribometer

1. Install chuck-motor subsystem.
   1. Level vibration isolation table. Place DC motor on the table; level the motor with shims and fix it with struts and bolts. Place support frame around the motor.
   2. Connect splined shaft to the motor shaft using a key. Put support plate on the frame with the splined shaft going through the hole in the plate. Set thrust needle-roller bearing on the support plate and around the splined shaft. Lubricate the bearing with cutting fluids.
   3. Connect the splined shaft to the chuck through an adapter plate, which has a splined shaft coupling on one side and the chuc....

Results

The representative measurements presented here were obtained from the modified tribometer shown in Figure 1. The piezoelectric actuator generates vibrations with amplitude of 2.5 μm at a frequency of 22 kHz. To study the dependence of friction and wear reduction on linear velocity, three different speeds (20.3, 40.6, and 87 mm/sec) were applied to the disc by changing the rotational speed of the motor. For all three groups, the number of disc revolutions and the travel distance of the pin were chosen as .......

Discussion

Experiments were conducted using this protocol to study the effect of linear speed on ultrasonic friction and wear reduction. The measurements show that ultrasonic vibrations effectively reduce friction and wear at three linear speeds. Consistent with prior observations, the amount of friction reduction decreases from 62.2% at 20.3 mm/sec to 29.3% at 87 mm/sec. Wear reduction is negligible with changing linear speed (45.8% to 48.6%).

Material properties such as Young’s modulus and y.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
DC Motor	Minarik	SL14
Electrical amplifier	AE Techron	LVC5050
Signal conditioner	Vishay Measurements Group	2310
Signal generator	Agilent	33120A
Piezoelectric stack	EDO corporation	EP200-62
Load cell	Transducer Techniques	MLP-50
Load sensor pad	FlexiForce	A201
Laser meter	Keyence corporation	LK-G32
Hall-effect probe and gaussmeter	Walker Scientific, Inc.	MG-4D
Data acquisition module	Data Physics	Quattro
Data acquisition software	Data Physics	SignalCalc Ace
Thermocouple reader	Omega	HH22
Optical profilometer	Bruker	Contour GT
Profilometer operation software	Bruker	Vision 64