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1.　Introduction

Conveyance systems with vacuum suction devices are

used to convey work items in devices for mounting elec-

tronic parts at specific locations on printed circuits and

devices for testing the electric performance of packaged

electronic parts. Air ejectors are often used as a vacuum

source for these devices.

One air ejector unit, consisting of an ejector, electro-

magnetic valves that turn suction on and off, vacuum sen-

sors, filters and other parts, is required for each suction

surface. For example, in a test device for packaged ICs,

sixteen suction pads (two sets of eight units) are allocated

to each block, where �� ICs are simultaneously delivered

for testing. The amount of compressed air required for

an ejector in this example is �00 (�� x ��) NL per minute,

indicating a large consumption of compressed air. Mean-

while, how to save energy has become a hotly debated is-

sue, and in recent years more manufacturers have begun

using mechanical-type vacuum pumps for energy-saving

purposes. ULVAC KIKO, Inc. plans to develop a series

of rocking piston type dry vacuum pumps with pumping

speeds ranging from �� to ��0 L per minute to meet the

growing need to save energy. This report describes the

structure and characteristics of the DOP-��0S vacuum

pump, which offers the highest pumping speed among

vacuum pumps in this series.

2.　 Energy-Saving Effects in Air Ejectors and Mechanical-type Vacuum Pumps

We will start with the basic principle of air ejectors.

An air ejector is a device used to generate vacuums by

using compressed air. Figure � shows the basic principle

of operation. As the figure shows, compressed air is sup-

plied through the throttled nozzle and discharged into

the diffusion chamber at high speed, and then expands

and flows into the diffuser. As compressed air flows into

the diffuser, the pressure in the diffusion chamber drops,

causing air to flow in from the suction port (�) for dis-

charge together with compressed air through the diffuser

and exhaust port.

Figure � shows an example of an air ejector unit.

The figure shows a typical suction conveyance system

using air ejectors. This system consists of an air com-

pressor and a dryer for producing compressed air, ejec-

tor units, electromagnetic valve for air pressure supply,

vacuum breaker valve, ejector, vacuum sensor, actuator,

suction pad and other miscellaneous parts.

Assuming a suction conveyance system equipped with

air ejectors for IC test equipment as shown in Figure �,

we will estimate its annual running cost and carbon diox-

ide emissions based on the amount of air and power con-

sumed by the air compressors.

The system assumed in our estimate consists of �� air

ejector units (four sets of eight units), each of which suc-

tions air inflow at �� NL per minute and consumes com-

pressed air at �� NL per minute, with ultimate pressure of

�� x �0� Pa. We assume that suction items weigh several

grams per piece and that the system is operated for �0

days per month, eight hours a day (of which four hours

are spent for suction). The amount of compressed air re-

quired to obtain the required air inflow is �� NL per min-

ute per unit, and since we have four sets of eight ejector

units, the total amount of compressed air supply required ＊　Research and Development Division, ULVAC KIKO, Inc.

Figure 1　Air Ejector

ULVAC TECHNICAL JOURNAL (ENGLISH) No.69E 2008

Development of a Rocking Piston Type Dry Vacuum Pump for Suction Conveyance

Hiroshi Hayashi*

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is �00 NL (� x � x ��) per minute. Given the number of

suction hours per day and operating hours per month,

the required amount of compressed air supply is �,��0

m� per month and ��,0�0 m� per year. Under these condi-

tions, the air compressors consume �.� kW of power, and

if operated for four hours per day using stagnating air, the

air compressors will consume �,��� kWh of power a year.

Assuming that power cost is �� yen per kWh, the annual

power cost will be ��,��� yen with annual carbon dioxide

emissions totaling �,��� kg (calculated based on 0.�� kg

of carbon dioxide emitted per kWh). (Of course, other

costs such as maintenance cost must also be considered.)

We also made estimates for mechanical-type vacuum

pumps. When assuming that air inflow of �� NL per

minute is produced per air ejector, past records lead us

to conclude that a mechanical-type vacuum pump with a

pumping speed of �00 L per minute will have sufficient

capability to produce air inflow equivalent to the total

amount produced by four sets of eight air ejector units,

even allowing for a �0% leeway. Therefore, the newly

developed rocking piston type dry vacuum pump (DOP-

��0S) has the capability required for the system. The

DOP-��0S consumes 0.�� kW of power and assuming

eight hours of operation a day, the annual power con-

sumption will amount to �,��0 kWh. Again assuming a

power cost of �� yen per kWh, the annual power cost will

total ��,��0 yen with carbon dioxide emissions of ��� kg

(calculated based on 0.�� kg of carbon dioxide emitted

per kWh).

The above results regarding the annual running cost

required for air ejectors and mechanical-type vacuum

pumps, and the resultant carbon dioxide emissions lead

us to conclude that using mechanical-type vacuum pumps

is likely to save energy consumption by approximately

��% compared with air ejectors, as well as reduce carbon

dioxide emissions.

3.　Mechanical-type Vacuum Pumps Suited to Suction Conveyance

Chapter � described the energy-saving effects of me-

chanical-type vacuum pumps for an IC test system. This

chapter describes the various types of mechanical-type

vacuum pumps that are expected to contribute to saving

energy.

3.1　Sliding Vane Type Dry Vacuum PumpFigure � shows a photo of a sliding vane type dry vacu-

um pump (DSB series); Figure � shows its structure.

As shown in Figure �, a sliding vane type dry vacuum

pump is an mechanical-type vacuum pump consisting of

a rotor, cylinder, vanes and other parts. This pump of-

Figure 3　Sliding Vane Type Dry Vacuum Pump

Figure 2　Example of an Air Ejector Unit

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fers the advantage of not using oil on sliding surfaces,

thereby providing a clean environment, and its multi-vane

structure enables stable pumping with little vibration. The

simple structure also enables high-speed pumping in low-

vacuum areas. Conversely, the pump is disadvantageous

in that its multi-vane structure continues to slide inside

the rotor and along the inner cylinder wall, thereby gen-

erating heat by friction and increasing power consump-

tion. The friction causes the vanes to wear down quickly,

making it necessary to perform maintenance about once

every �,000 hours.

3.2　Diaphragm Type Dry Vacuum PumpFigure � shows a photo of a diaphragm type dry vacu-

um pump; Figure � shows its structure.

A diaphragm type dry vacuum pump is a mechanical-

type vacuum pump that drives air via the pumping move-

ment of a diaphragm attached to a connecting rod, which

in turn is affixed to an eccentric rotary shaft. This pump

offers several advantages such as making little operating

Figure 5　Diaphragm Type Dry Vacuum Pump Figure 6　Structure

noise, a simple structure that is easy to maintain, rela-

tively long service life, and providing a clean environment

since it needs no oil. Its disadvantage is the diaphragm’s

restricted pumping movement, which makes it difficult to

increase pumping speed.

3.3　Rocking Piston Type Dry Vacuum PumpA rocking piston type dry vacuum pump is a mechan-

ical-type vacuum pump that drives air via the pumping

movement of a connecting rod af fixed to an eccentric

rotary shaft, just like a diaphragm type dry vacuum pump.

This pump uses no oil on sliding surfaces, and thus pro-

vides a clean environment. The pump also has features

a simple, easy to maintain structure, and since it moves

with a longer stroke than a diaphragm type pump, its size

can be reduced while at the same time increasing pump-

ing speed. This pump provides ultimate pressure approxi-

mating that of a diaphragm type pump and achieves stable

performance at low pressure. Unlike sliding vane type

dry vacuum pumps, vanes do not get broken due to the

adhesion of powder caused by friction, and the abrasion

of sealing material can be easily prevented by monitoring

the pressure. Market records show that the service life of

sealing material used in a rocking piston type dry vacuum

pump is around �0,000 hours. The disadvantage of this

pump is that it generates louder operating noise than a

diaphragm type dry vacuum pump.

4.　 Development of a Suction Conveyance Pump Series

ULVAC KIKO, Inc. is developing a series of rocking pis-

ton type dry vacuum pumps for various systems, such as

small-size electronic parts mounters, circuit board print-

Figure 4　Structure Diagram

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top of the cylinder, along with a pump head separating the

suction chamber and exhaust chamber. A gasket and a

pump head cover are attached to the pump head.

Air evacuation movement is produced by the rotating

eccentric rotary shaft connected directly to the motor,

which causes the connecting rod to move up and down in-

side the cylinder. The connecting rod, linked to the rotary

shaft at a single point, makes a rocking movement as it

moves up and down. The connecting rod movement from

the top dead point to the bottom dead point closes the

exhaust valve and opens the suction valve, thereby allow-

ing an intake of air through the suction port. Conversely,

connecting rod movement from the bottom dead point to

Figure 8　Principle of Movement

ers, taping equipment and mechanical-type packaging

machines.

Four types of pumps with pumping speeds ranging

from �� L per minute to ��0 L per minute have been de-

veloped, with power source specifications appropriate for

overseas use.

Figure � shows the lineup of our rocking piston type

dry vacuum pumps; Table � lists the major pump specifi-

cations.

5.　 Principles of Rocking Piston Type Dry Vacuum Pumps

This chapter describes the structure and principles of

rocking piston type dry vacuum pumps.

Figure � shows the structure of a rocking piston type

dry vacuum pump. As shown in the figure, a rocking pis-

ton type dry vacuum pump has an eccentric rotary shaft

around the motor shaft, with a bearing and a connecting

rod affixed to the rotary shaft. A sealing disc made of spe-

cial resin (called cup packing) is affixed to the upper part

of the connecting rod, in contact with the inner cylinder

wall. A suction valve and exhaust valve are installed on

Figure 7　Development of a Series of Rocking Piston Type Dry Vacuum Pumps

Table 1　Major Pump Specifications

Model namePumping speed

(L/min)Ultimate pressure(x 103Pa)50Hz 60Hz

DOP-90S 85 95 19.0DOP-180S 170 190 19.0DOP-300S 300 330 8.0DOP-420S 420 460 17.3

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6.2　Characteristics1) Three options are available for choosing the loca-

tion of the suction port in line with the layout of system piping.

When installing a pump in a system, it is often neces-

sary to install the pump in an area where electric parts

and pipes are concentrated, necessitating such adjust-

ment at installation as attaching a special connector to

the suction port according to the layout of system pip-

ing. The newly developed pump is designed to provide

three options for the suction port location in order to

match a wider range of layouts and facilitate piping in-

stallation.

2) The structure was redesigned to improve maintain-ability.

Pumps developed in the past were designed with a

motor set between compression chambers, as shown in

Figure ��. Due to this structure, joints and pipes were

needed to connect the chambers, but then had to be

removed at maintenance and for replacing consumable

parts like O-rings. To avoid this burden, we redesigned

the arrangement of the motor and compression cham-

the top dead point closes the suction valve and opens the

exhaust valve, thereby discharging air from the exhaust

port. This cycle is repeated according to rotation of the

motor.

6.　 Structure and Characteristics of the DOP-420S

6.1　Internal Structure of the DOP-420S PumpFigure � shows the internal structure of the DOP-��0S

pump.

This pump consists of (�) a motor, (�) casing, (�) ec-

centric rotary shafts, (�) connecting rods, (�) cylinders,

(�) suction valves, (�) exhaust valves, (�) pump heads, (�)

gaskets and (�0) pump head covers, with four compres-

sion chambers horizontally interconnected. Piping con-

necting these chambers is incorporated into the structure

of the casing and pump head parts. Thus, the pump is

designed to eliminate connecting pipes and joints so as to

reduce the number of parts and improve maintainability.

Figure 9　Internal Pump Structure

Figure 10　Appearance

Figure 11　Parallel Movement

Figure 12　Counter Movement

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bers as shown in Figure �� to eliminate connectors and

pipes, thereby improving maintainability and reducing

the number of parts, including connectors and pipes.

3) The pump is designed to produce counter piston movement so as to reduce vibration.

As shown in Figure ��, piston pumps developed in

the past were designed so that as connecting rods �a

and �b on the left move in the arrow-indicated direc-

tion, and connecting rods �a and �b on the right move

in the opposite direction, thereby making it necessary

to attach balance weights to reduce vibration caused by

such movement. In contrast, the DOP-��0S (Figure ��)

is designed so that connecting rods ��a and ��b each

move in directions away from the motor shaft, while

connecting rods ��a and ��b each move in directions

toward the motor shaft, thereby canceling out vibration

to achieve a low level of vibration.

7.　 Performance Characteristics of the DOP-420S

7.1　Pumping Speed CurveFigure �� shows the pumping speed curve of the DOP-

��0S.

Pumping speed decreases as the suction pressure

drops. Although the amount of free air leaking through

the cup packing remains constant regardless of changes

in suction pressure, the effect of this incoming air be-

comes more evident as the suction pressure drops—the

reason why pumping speed decreases. The decrease is

also caused by a dead volume at the top dead point of the

piston. This does not pose any serious problem with the

newly developed pump, which is designed for the purpose

of suction conveyance.

7.2　Power ConsumptionFigure �� shows the power consumption of the DOP-

��0S.

Power consumption is at the lowest level when ultimate

pressure is achieved, reaching a peak around �0 x �0� Pa

where mechanical loss and pressure loss are greatest.

7.3　 Comparison of Noise Levels with a Sliding Vane Type Dry Vacuum Pump

Figure �� compares noise levels between the DOP-��0S

and a sliding vane type dry vacuum pump (at �0 Hz).

With a rotary dry vacuum pump, the noise level shows

a moderate increase from ultimate pressure to �0 x �0� Pa,

subsequently showing a gradual decrease until �0 x �0�

Pa, and then increasing again toward the level of air pres-

sure. This is caused by an increase in airflow noise.

With the DOP-��0S, the noise level is lowest when ulti-

mate pressure is achieved, remaining almost flat after the

pressure exceeds �0 x �0� Pa. Compared with the sliding

vane type dry vacuum pump, the noise level is consistent-

ly lower by � to � dB (A).

Figure 15　Noise Level

Figure 13　Pumping Speed Curve

Figure 14　Power Consumption

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8.　 Service Life of Consumable Parts and Maintainability

8. 1　Service Life of Consumable PartsThe service life of consumable parts required for the

rocking piston type dry vacuum pump largely depends

on that of the cup packing (sealing material) that slides in

contact with cylinders. Cup packing is normally made of

fluoride resin and the matching parts are made of anode-

oxidized aluminum or stainless steel. The same standards

for the DOP-�00S (developed earlier) concerning ma-

terials for cup packing (related to consumables) and its

matching parts, sliding distance and rotational velocity

were adopted for the design of the DOP-��0S. Figure ��

shows measurements taken every �,000 hours to evaluate

the stability of ultimate pressure of the DOP-�00S, which

served as standards for the DOP ��0S. Pumps were oper-

ated under two different conditions: continuous operation

under ultimate pressure and operation under variable

pressure. The criterion for ultimate pressure of the DOP-

�00S was less than � x �0� Pa. With the pump operated

continuously under the variable pressure condition, the

pressure began rising after about ��,000 hours, exceed-

ing the criterion after about ��,�00 hours. In contrast,

with the pump operated continuously under the ultimate

pressure condition, the pressure began rising after about

��,000 hours, exceeding the criterion after about �0,000

hours. When the criterion was exceeded, the cup packing

in contact with the cylinder had lost about ��% of its initial

weight due to abrasion. These results lead us to conclude

that the appropriate service life for cup packing is about

��,000 hours.

The DOP-��0S has been operated for more than �,000

hours without any increase in ultimate pressure. Cup

packing lost about �0% of its initial weight on the surface

in contact with the cylinder after �,000 hours of operation,

which makes it likely that it will last about ��,�00 hours

before its weight falls to the same level as that of the DOP-

�00S.

8.2　MaintainabilityAs Figure �� shows, the new vacuum pump is designed

so that cup packing (sealing material)—a consumable—can be easily and quickly replaced without using special

tools by simply loosening the hexagon socket cap screws

Figure 17　Replacement

Figure 16　Stability of Ultimate Pressure

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used to affix the pump head and cup packing retainers.

9.　Summary

Since about five years ago, we have been recommend-

ing that the system manufacturers of electronic parts

mounters and IC test equipment use mechanical-type

vacuum pumps to save energy, but few manufacturers

have accepted our proposal because its initial cost would

be high. Nevertheless, more system manufacturers have

begun using our series of rocking piston type dry vacuum

pumps over the past two years. We believe that this is due

to the growing demand for saving energy among equip-

ment end users.

A mechanical-type vacuum pump requires a structure

connecting a mobile pump head attached with a suction

pad to the pump via movable vacuum piping, and since air

is repeatedly evacuated with pressures ranging from �00

to �0 x �0� Pa, the air flow within the vacuum piping rang-

es between turbulent and laminar flows. Consequently,

there are areas where ordinary conductance calculations

as used in vacuum technology do not apply. System manu-

facturers must develop the necessary know-how and skills

regarding piping. Our company provides various services

to help choose pumps of different types, including per-

forming tests using actual production machines.