Lab 11: Automatic Lubrication Shot

Application Scenario

A CNC machining center has slideways and ball screws that must be oiled regularly. Many machines use an automatic lubrication pump that delivers a measured "shot" of oil every time the machine starts. The pump must run for a fixed time — long enough to push oil to every bearing, but not so long that oil floods the machine or the pump overheats.

The sequence is:

1. The operator presses START. The machine starts, and the lube pump starts at the same moment.
2. After the set time (4 seconds in this lab), the lube pump stops by itself. The machine keeps running.
3. Every new start gives a new lube shot.

The device that turns an output ON for a fixed time is an interval timer (also called an interval-on or one-shot timer). In this lab, G1 shows the MACHINE running and R1 shows the LUBE PUMP running.

Learning Outcomes

After this lab you will be able to:

• Explain how an interval timer (TR3) behaves.
• Compare interval, on-delay, and off-delay timers on a timing diagram.
• Use an interval timer to produce a fixed-length output pulse from a latched signal.

Required Components

Background

How TR3 (interval) works. TR3 is wired like TR1: coil 2–7 (pin 7 to L2), contacts TR3A (1-3-4) and TR3B (8-6-5). Its contacts behave differently:

• Coil energized: the contacts change state immediately (N.O. closes, N.C. opens) and the timer starts counting.
• Set time reached: the contacts return to normal, even though the coil is still energized.
• Coil de-energized: the contacts return to normal immediately (if they have not already), and the timer resets. The next time the coil is energized, a new interval begins.

In other words, an interval timer turns a long ON signal into a short, fixed ON pulse at the beginning.

Three timers compared. All three are controlled by the same kind of ON/OFF input:

Timing Diagram

• At each start, CR1 seals in and the TR3 coil is energized. TR3A closes immediately and the lube pump (R1) runs.
• At shot done (4 s later), TR3A opens and the lube pump stops. The machine (G1) keeps running.
• At stop, CR1 and the TR3 coil drop out, and TR3 resets. The next start gives a new, full 4-second shot.

Functional Requirements

1. Pressing PB1 starts the machine (CR1, G1) and the lube pump (R1) at the same time.
2. The lube pump stops by itself after 4 seconds. The machine keeps running.
3. Pressing PB2 stops the machine. If PB2 is pressed during the lube shot, the pump stops too.
4. Every new start gives a full 4-second lube shot.
5. After a power outage, nothing restarts by itself.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. L1 (+24 V) is the left rail and L2 (GND) is the right rail. Use pencil. This design uses 5 grid lines.

Step 1 — Rungs 1–2 (grid lines 1–2): machine start/stop

1. On grid line 1, draw PB2 (N.C.), PB1 (N.O.), and the CR1 coil (2–7). Mark N2 (between PB2 and PB1) and N3 (between PB1 and the coil).
2. On grid line 2, draw the CR1A N.O. seal-in (1–3) from N2 to N3.

Step 2 — Rung 3 (grid line 3): the interval timer coil

1. From node N3, draw a vertical line down to grid line 3.
2. Draw the TR3 coil (pins 2 and 7) and connect pin 7 to L2.

Step 3 — Rung 4 (grid line 4): lube pump

1. From node N3, continue the vertical line down to grid line 4.
2. Draw a TR3A N.O. contact (pins 1 and 3), then R1. Connect R1 to L2.
3. Mark the node between TR3A and R1 as N4.

Step 4 — Rung 5 (grid line 5): MACHINE running lamp

1. Starting from L1, draw a CR1B N.O. contact (pins 8 and 6), then G1. Connect G1 to L2.
2. Mark the node between CR1B and G1 as N5.

Step 5 — Trace your drawing with a pencil

1. PB1 has just been pressed (t = 1 s). Which coils are energized? Are R1 and G1 ON?
2. t = 6 s. Is the TR3 coil still energized? Is TR3A open or closed? What are R1 and G1 doing?
3. PB2 is pressed at t = 2 s, during the lube shot. What happens to R1? What happens at the next START?
4. Compare with Lab 08: if TR3 were replaced by TR1 (on-delay), when would R1 turn ON and OFF?

Step 6 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

Step 1 — List every terminal on each node

Write down every terminal that touches each node, with the device name. The N2 row is done for you.

Step 2 — Chain the terminals of each node together

A node with k terminals needs k − 1 wires, connected in a chain. Keep no more than two wires under any terminal screw.

Step 3 — Fill in the wiring table on the worksheet

Step 4 — Get instructor approval

Show your ladder diagram, contact allocation table, node table, and wiring table to the instructor. Do not start wiring until the worksheet is stamped.

Key Wiring Points

1. An interval timer's N.O. contact closes at once and opens after the set time, while the coil stays energized.
2. Energize the interval timer from a latched node (N3). The output pulse length is then set by the timer, not by how long the operator holds the button.
3. Feed the output from the latched node too, so STOP always ends the pulse.
4. TR3 has the same pin numbers as TR1 and CR1. Always write TR3:3, never just 3.

Wiring Procedure

1. Power OFF the trainer. Set TR3 to 4 seconds.
2. Wire the circuit from your approved wiring table, one node at a time. Mark each wire as you install it.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR1:1 should read near 0 Ω, and high while PB2 is pressed.
   • CR1:1 to CR1:2 should read high, and near 0 Ω while PB1 is pressed.
   • CR1:2 to TR3:2 and CR1:2 to TR3:1 should read near 0 Ω (all on N3).
   • TR3:1 to TR3:3 should read high (timer OFF).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Describe what TR3's N.O. contact does when the coil is energized, after the set time, and when the coil is de-energized.
2. Fill in the blanks: an on-delay timer delays the ______ of the output; an off-delay timer delays the ______ of the output; an interval timer limits the ______ of the output.
3. Why is the TR3 coil connected to N3 instead of directly after PB1? What would the lube shot look like if the operator tapped PB1 quickly?
4. Why is it important that the lube pump runs for a limited time, instead of for as long as the machine runs?
5. The machine runs for 8 hours without stopping. How many lube shots does it get? Why might this be a problem, and what kind of timer circuit would solve it?

Applying This Circuit Pattern

The fixed-length pulse pattern: a latched or long signal energizes an interval timer, and its N.O. contact gives a short output of a fixed length at the beginning.

Design question for the future: Some machines must NOT start their main motor until the lube shot is finished (pre-lubrication). Which TR3 contact could hold the main motor OFF during the shot and release it afterward? (You will try this in the Extension.)

Extension: Pre-Lubrication

Change the circuit so that the machine motor waits until the lube shot is finished:

• Keep CR1 as the start/stop latch (now a "start request" relay).
• Add CR2 as the machine motor contactor (coil 14–13).
• Feed CR2 from N3 through TR3B. Which TR3B pins do you use: 8–6 or 8–5?
• Move G1 so it shows the motor (CR2) running.

Draw the new rungs, wire them, and test: after START, R1 should be ON for 4 s, then R1 OFF and G1 ON.