Pulley Replacement & Operating Pressure Change

Applies to: South-Tek STS Series belt-driven rotary screw compressors (STS-5 through STS-100). Does not apply to direct-drive or VSD models (STSD / STSP / STSP2), which change pressure through the controller rather than by swapping pulleys.

On belt-driven STS compressors, the operating pressure is set by the drive ratio between the motor pulley and the airend pulley. This article covers removing and replacing those pulleys — either to change the unit's operating pressure or to replace a damaged pulley — and re-tensioning the belts to the correct value for the new configuration.

⚠ Warning — Stored Energy Before opening any guard or panel, stop the compressor, disconnect and lock out the main power, and verify the unit is fully depressurized. Only qualified personnel should perform this procedure. See Safety Precautions & Symbols.

Before You Begin

Confirm your target operating pressure and look up the required motor pulley, airend pulley, belt type, and belt tension in the Pulley Selection Tables below. Order the correct pulleys and belts by their stock numbers before starting. Have on hand:

• A pulley puller / removal tool (the pulley face has two guided holes that accept a puller)
• A torque wrench or impact wrench for the tension and chassis bolts
• A steel ruler or laser belt-alignment tool for setting pulley parallelism
• A belt-tension gauge

Replacement Procedure

1. Remove the belt guard. Remove the belt-guard side connecting bolts and lift the guard away.

[ FIGURE I — Remove the belt guard ]

2. Relieve the motor belt tension. Open the cover on the side where the motor terminal box is located. Loosen the motor plate's chassis connection bolts, then back off the motor tension-plate bolts in the removal direction with a wrench or torque gun. As the tension plate moves toward the screw (airend) side, the belt goes slack. Confirm the belt is loose enough to remove.

[ FIGURE II — Relieve motor tension ]

3. Remove the pulley front washer. On whichever pulley you are changing (motor or airend, per the Selection Tables), unscrew the bolt holding the front washer and remove the washer.

[ FIGURE III — Remove pulley front washer ]

4. Pull the pulley. Remove the pulley using a puller / removal tool. The two guided holes on the pulley face are provided for pulling and can be used for removal.

[ FIGURE IV — Pull the pulley ]

5. Install the new pulley and belts. Fit the new pulley, then seat the new belts in the pulley channels. Gradually tighten the motor tension-table bolts in the tightening direction to pre-position the pulleys.

[ FIGURE V — Install pulley & tension belts ]

6. Align the pulleys. Set the pulleys parallel using a steel ruler or laser belt-alignment device. Use the two tension-table bolts to adjust parallelism.

7. Set belt tension and lock down. Tension the belts to the Static Tension per Belt (N) value listed in the Selection Tables for your model and pressure, and take measurements to confirm. Re-check parallelism after tensioning. Tighten the counter nuts on the tension bolts so vibration cannot loosen them, and confirm the motor tension-table and upper-chassis connection bolts are tight.

▲ Attention — Re-check After Break-In After 25–50 working hours on the new belts and pulleys, re-check belt tension. If the belts have loosened, re-tension them using the tension bolts.

Pulley Selection Tables

First find your model in the reference table for its fixed values, then go to your target operating pressure for the pulley, belt, and tension data. Values are from manufacturer document MT-144 (Rev.0).

Model reference (all pressures)

Model	Airend	Motor RPM	Motor Power (kW)
STS-5	SCA7	3480	4
STS-7.5	SCA7	3516	5.5
STS-10	SCA8	3516	7.5
STS-15	SCA8	3534	11
STS-20	SCA9	3534	15
STS-25	SCA9	3534	18.5
STS-30	SCA10	3546	22
STS-40	SCA10	3552	30
STS-50	SCA13	3552	37
STS-60	SCA13	3558	45
STS-75	SCA14	3570	55
STS-100	SCA14	3574	75

Target pressure: 100 PSI

Pulley & belt selection

Model	Motor Pulley	Motor Pulley Stock #	Airend Pulley	Airend Pulley Stock #	Belt
STS-5	SPZ 160-01	VS-3132.11	SPZ 110.72-01	VS-3160.09	XPZ 1187
STS-7.5	SPZ 160-01	VS-3132.12	SPZ 84.22-01	VS-3150.12	XPZ 1137
STS-10	SPZ 200-01	VS-3137.10	SPZ 141.86-01	VS-3162.11	XPZ 1312
STS-15	SPZ 200-02	VS-3137.12	SPZ 98.15-02	VS-3151.12	XPZ 1312
STS-20	SPZ 200-02	VS-3137.12	SPZ 153.35-02	VS-3169.31	XPZ 1412
STS-25	SPZ 170-03	VS-3134.10	SPZ 107.32-03	VS-3175.07	XPZ 1562
STS-30	SPZ 190-03	VS-3136.10	SPZ 140.68-03	VS-3181.09	XPZ 1600
STS-40	SPZ 300-03	VS-3145.00	SPZ 163.64-03	VS-3257.06	XPZ 1762
STS-50	SPA 224-03	VS-3103.10	SPA 167.29-03	VS-3194.08	XPA 1700
STS-60	SPB 250-03	VS-3105.50	SPB 155.94-03	VS-3263.53	XPB 1800
STS-75	SPB 250-03	VS-3106.50	SPB 196.63-03	VS-3208.55	XPB 2000
STS-100	SPB 300-04	VS-3108.10	SPB 179.84-04	VS-3215.04	XPB 2000

Setup & performance

Model	Air Flow (m³/min)	Airend RPM	Centres (mm)	Static Tension / Belt (N)
STS-5	0.67	5029	381	218
STS-7.5	0.91	6680	375	274
STS-10	1.22	4957	387	386
STS-15	1.82	7201	418	276
STS-20	2.49	4609	428	332
STS-25	3.01	5598	562	293
STS-30	3.85	4789	540	327
STS-40	5.18	6512	512	456
STS-50	6.3	4765	542	566
STS-60	7.6	5704	579	861
STS-75	9.55	4539	649	937
STS-100	12.5	5962	620	1108

Target pressure: 125 PSI

Pulley & belt selection

Model	Motor Pulley	Motor Pulley Stock #	Airend Pulley	Airend Pulley Stock #	Belt
STS-5	SPZ 160-01	VS-3132.11	SPZ 124.42-01	VS-3160.08	XPZ 1187
STS-7.5	SPZ 160-01	VS-3132.12	SPZ 93.56-01	VS-3150.11	XPZ 1137
STS-10	SPZ 160-01	VS-3132.12	SPZ 126.67-01	VS-3162.10	XPZ 1237
STS-15	SPZ 200-02	VS-3137.12	SPZ 108.54-02	VS-3151.11	XPZ 1312
STS-20	SPZ 200-02	VS-3137.12	SPZ 171.55-02	VS-3169.29	XPZ 1412
STS-25	SPZ 170-03	VS-3134.10	SPZ 118.29-03	VS-3175.06	XPZ 1562
STS-30	SPZ 190-03	VS-3136.10	SPZ 156.65-03	VS-3181.08	XPZ 1600
STS-40	SPZ 300-03	VS-3145.00	SPZ 182.62-03	VS-3257.05	XPZ 1800
STS-50	SPA 224-03	VS-3103.10	SPA 183.24-03	VS-3194.09	XPA 1700
STS-60	SPB 250-03	VS-3105.50	SPB 170.83-03	VS-3263.54	XPB 1800
STS-75	SPB 250-03	VS-3106.50	SPB 217.47-03	VS-3208.56	XPB 2000
STS-100	SPB 300-04	VS-3108.10	SPB 195.94-04	VS-3215.05	XPB 2000

Setup & performance

Model	Air Flow (m³/min)	Airend RPM	Centres (mm)	Static Tension / Belt (N)
STS-5	0.57	4475	370	218
STS-7.5	0.8	6013	368	273
STS-10	1.07	4441	393	345
STS-15	1.62	6512	411	275
STS-20	2.2	4120	433	331
STS-25	2.71	5079	554	293
STS-30	3.43	4301	528	326
STS-40	4.64	5835	518	456
STS-50	5.67	4342	530	566
STS-60	6.86	5207	568	860
STS-75	8.54	4104	633	937
STS-100	11.37	5472	608	1107

Target pressure: 150 PSI

Pulley & belt selection

Model	Motor Pulley	Motor Pulley Stock #	Airend Pulley	Airend Pulley Stock #	Belt
STS-5	SPZ 132-01	VS-3132.10	SPZ 114.07-01	VS-3160.11	XPZ 1137
STS-7.5	SPZ 160-01	VS-3132.12	SPZ 103.89-01	VS-3150.14	XPZ 1137
STS-10	SPZ 160-01	VS-3132.12	SPZ 138.43-01	VS-3162.14	XPZ 1237
STS-15	SPZ 200-02	VS-3137.12	SPZ 118.06-02	VS-3151.14	XPZ 1312
STS-20	SPZ 170-02	VS-3138.12	SPZ 161.28-02	VS-3169.33	XPZ 1387
STS-25	SPZ 170-03	VS-3134.10	SPZ 129.84-03	VS-3175.10	XPZ 1562
STS-30	SPZ 190-03	VS-3136.10	SPZ 172.67-03	VS-3181.12	XPZ 1600
STS-40	SPZ 300-03	VS-3145.00	SPZ 201.82-03	VS-3257.09	XPZ 1800
STS-50	SPA 224-03	VS-3103.10	SPA 199.61-03	VS-3194.10	XPA 1700
STS-60	SPB 184.98-03	VS-3106.53	SPB 137.4-03	VS-3263.55	XPB 1650
STS-75	SPB 240-03	VS-3106.51	SPB 230.32-03	VS-3208.57	XPB 2000
STS-100	SPB 250-04	VS-3108.11	SPB 177.35-04	VS-3215.06	XPB 2000

Setup & performance

Model	Air Flow (m³/min)	Airend RPM	Centres (mm)	Static Tension / Belt (N)
STS-5	0.5	4027	375	226
STS-7.5	0.7	5415	360	273
STS-10	0.96	4064	384	344
STS-15	1.47	5987	404	275
STS-20	1.97	3725	433	337
STS-25	2.45	4627	545	292
STS-30	3.07	3902	515	326
STS-40	4.17	5280	504	455
STS-50	5.13	3986	517	565
STS-60	6.24	4790	574	745
STS-75	7.66	4104	631	913
STS-100	10.37	5038	663	948

Target pressure: 190 PSI

Pulley & belt selection

Model	Motor Pulley	Motor Pulley Stock #	Airend Pulley	Airend Pulley Stock #	Belt
STS-5	SPZ 132-01	VS-3132.10	SPZ 131.25-01	VS-3160.10	XPZ 1137
STS-7.5	SPZ 160-01	VS-3132.12	SPZ 118.43-01	VS-3150.13	XPZ 1162
STS-10	SPZ 160-01	VS-3132.12	SPZ 155.96-01	VS-3162.12	XPZ 1262
STS-15	SPZ 200-02	VS-3137.12	SPZ 131.3-02	VS-3151.13	XPZ 1337
STS-20	SPZ 170-02	VS-3138.12	SPZ 186.52-02	VS-3169.32	XPZ 1387
STS-25	SPZ 170-03	VS-3134.10	SPZ 149.48-03	VS-3175.08	XPZ 1600
STS-30	SPZ 190-03	VS-3136.10	SPZ 194.16-03	VS-3181.10	XPZ 1612
STS-40	SPZ 300-03	VS-3145.00	SPZ 227.4-03	VS-3257.07	XPZ 1850
STS-50	SPA 190-03	VS-3101.10	SPA 191.08-03	VS-3194.11	XPA 1700
STS-60	SPB 184.98-03	VS-3106.53	SPB 154.68-03	VS-3263.56	XPB 1700
STS-75	SPB 185-03	VS-3106.55	SPB 205.24-03	VS-3208.58	XPB 1900
STS-100	SPB 250-04	VS-3108.11	SPB 200.97-04	VS-3215.07	XPB 1900

Setup & performance

Model	Air Flow (m³/min)	Airend RPM	Centres (mm)	Static Tension / Belt (N)
STS-5	0.4	3500	362	225
STS-7.5	0.59	4750	362	273
STS-10	0.82	3607	383	344
STS-15	1.29	5383	407	275
STS-20	1.65	3221	413	337
STS-25	2.09	4019	549	292
STS-30	2.66	3470	505	326
STS-40	3.66	4686	510	455
STS-50	4.45	3532	551	550
STS-60	5.45	4255	573	745
STS-75	6.51	3218	643	848
STS-100	9.02	4446	595	948

Target pressure: 218 PSI

Pulley & belt selection

Model	Motor Pulley	Motor Pulley Stock #	Airend Pulley	Airend Pulley Stock #	Belt
STS-5	—	—	—	—	—
STS-7.5	—	—	—	—	—
STS-10	SPZ 150-01	VS-3132.14	SPZ 183.76-01	VS-3162.13	XPZ 1262
STS-15	SPZ 200-02	VS-3137.12	SPZ 145.61-02	VS-3156.08	XPZ 1337
STS-20	SPZ 139.15-02	VS-3138.13	SPZ 168.12-02	VS-3169.30	XPZ 1337
STS-25	SPZ 170-03	VS-3134.10	SPZ 164.42-03	VS-3175.09	XPZ 1600
STS-30	SPZ 170-03	VS-3134.10	SPZ 193.52-03	VS-3181.11	XPZ 1612
STS-40	SPZ 224-03	VS-3139.11	SPZ 187-03	VS-3257.08	XPZ 1700
STS-50	SPA 190-03	VS-3101.10	SPA 207.78-03	VS-3194.12	XPA 1700
STS-60	SPB 184.98-03	VS-3106.53	SPB 167.68-03	VS-3263.57	XPB 1700
STS-75	SPB 185-03	VS-3106.55	SPB 226.96-03	VS-3208.59	XPB 2000
STS-100	SPB 212-04	VS-3108.12	SPB 200.97-04	VS-3215.07	XPB 1900

Setup & performance

Model	Air Flow (m³/min)	Airend RPM	Centres (mm)	Static Tension / Belt (N)
STS-5	—	—	—	—
STS-7.5	—	—	—	—
STS-10	0.61	2870	369	354
STS-15	1.12	4854	397	275
STS-20	1.46	2925	427	263
STS-25	1.87	3654	538	292
STS-30	2.32	3115	520	331
STS-40	3.27	4255	527	402
STS-50	4.01	3248	538	551
STS-60	4.94	3925	573	745
STS-75	5.79	2910	676	849
STS-100	8.13	4057	626	876

Related Articles

• V-Belt Inspection & Replacement
• STS Maintenance Schedule
• Spare Parts Ordering

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